Hi xSTLx, if you're serious about building a quiet computer I suggest you take a look at SPCR. You can easily lose a few days (or weeks) there if you haven't visited before, you'll also find spod there in the forums.
If you decide you want a quiet PC then it's best to start with that aim rather than quietening a loud one afterwards. A quiet PC is all about air flow (unless you go water, but thatâ€™s harder and more expensive and, IMO, only worth considering for the very top end SLI/crossfire systems), a fully passive PC is the holy grail but MUCH harder then a very quiet air cooled PC. How quiet can a PC with fans be? Well my PC has 4 80mm fans (Panaflow L1As), one intake, two exhaust and another in the PSU, it also has 92mm fan in the Zalman CPU cooler and another in the VGA Silencer. With all the case fans turned down to 5v (~1000rpm) and the CPU cooler also ~1000rpm the main noise source it the hard drive. It's about as loud as the hum from the monitor and a car on the road 100m away (it's midnight in a rural location) drowns it out, it's seriously quiet. What I'm trying to say is with good air flow paths and good fans (properly quiet ones not just any that claim quiet/silent) you can have a very quiet PC. Beware the silencing game can become very addictive
More general comments: at this time AMD is preferred over Intel, faster chips and much easier to cool quietly. Definitely buy dual core, it's the way of the future, more and more software will take advantage of it, I'm told a dual core machine has a "feel" and responsiveness, especially under load, that single core can't match.
With regard to cache memory I'm not sure, different people and different benchmarks tell different stories. This review shows a Sempron (single channel s754) with 128KB cache overclocked to 2.6GHz being about 5% slower than FX57 (2.6GHz, dual channel memory, 1MB cache) in gaming benchmarks. When you consider the FX57 is ~TEN TIMES the price If I was buying I would probably aim for 1MB cache CPU anyway...just to be sure!
Not very up on graphics at the moment, there are just so many, older ones, newer ones at all different price points. I would suggest going for Nvidia 7x00 or ATI X1x00 as they are the latest and support all the latest features, which may become more important than a bit of extra speed from an older but equally priced model. The 7600 series looks like itâ€™s going to offer a good balance of performance, features and low power consumption, which is easier to cool quietly/passively. If you have any intension of ever having two graphics cards, maybe as an upgrade path be aware that ATI crossfire requires an ATI (or Intel not that it's relevant to you) chipset and Nvidia SLI requires an Nvidia chipset, it's unlikely they will ever allow each others dual cards to work with the others chipset. For single cards it doesn't matter.
The nforce 4 is the current favourite chipset for s939 but the latest ATI chipsets also seam competitive and they are lower power meaning much easier to cool with a simple northbridge heatsink rather than needing a whining little fan. Also the fact the nforce 4 chipset is placed under the graphics cards further complicates the quiet cooling of it, but it can be done. The integrated memory controller is a great performance equaliser so ULi or even VIA chipsets don't really give anything up in performance if you happened to be tempted by a board with such a chipset. www.anandtech.com have reviewed most of the available chipsets, CPUs, graphics cards etc. Probably worth your while spending a few hours digging through the reviews there.
I agree with you the Tenor case is very pretty but I have reservations about it as the base for a quiet PC. 2000rpm 80mm and 2500rpm 60mm fans are not silent, they won't even rate as quiet by SPCR standards. It's the 60mm fans that worry me most, there aren't any good options for moving enough air through that size fan. Possibly slowing the fans down and using a PSU with 120mm fan to increase the exhaust would work. It doesn't look like there is room to cut the case to take 80mm fans in place of the 60mm ones, it might be possible to cut the grills out of the holes (for best airflow) and mount two 80mm fans out the back of the case either using standard adapters if they fit or a custom made one. (I've used thin card and stick tape as an easy and effective way of making ducts and adapters!)
If you do go with the Tenor case don't use a passive PSU, do use a 120mm fan one as you need it's exhaust contribution, the Seasonic S12 series is the current favourite.
You haven't mentioned a hard drive at all that I noticed, serious silencers find 3.5" HDDs too loud so they use 2.5" laptop HDDs Many people going down the quiet route eventually find they need to decouple the HDD from the chassis to avoid amplifying it's hums and rattles. I would recommend a single HDD of whatever size you need as the quietest option against two or more smaller (possible each slightly quieter) drives. If you need huge amounts of storage then consider a network server that can be hidden out of ear-shot.
I would get at least 1GB of RAM, possibly 2GB if the budget will handle it.
Not sure if it applies to you but advice I give to people buying a complete new system is don't skimp on the monitor(s), don't skimp on the monitor(s) and don't skimp on the mouse & keyboard. They are the bits of the PC you interface with and have a large impact on how you feel about it and how nice it is to use.
I'll stop rambling now, good luck, Seb