For a case to reduce vibration, you want the thickest "Cold Rolled Steel" you can get.
The Antec NSK3480 is only $100, and comes with an 80+ certified PSU and is made of .8mm CRS. Thats pretty thick.
I think the Antec Solo is made from 1.0mm CRS, not sure though.
You may also want to look into getting different hard drives. Heres a listing of quiet hard drives tested by SPCR:
Theirs a column for the amount of vibration the drive creates. the WDGP is the best 3.5" drive as far as vibration is concerned, though it does only spin at 5400rpm. Next up below that are the WD Caviar drives followed by the Samsung F1.
If you really want zero vibrations though, get a 2.5" drive. The velociraptor is a 2.5" drive with very good performance and virtually no vibration or noise. If you want something less expensive than that, look at any of the 7200rpm 2.5" drives. 5400rpm drives are going to be the most quiet though but will have slower performance. But thats always the trade-off. If you want something truely silent with no vibration its probably going to run slow, and things that run fast typically either cost a lot or make a lot of noise/vibrations.