I don't know where you get the info from, but I have an Extreme IV in my lab right now that works as a real HD running windows XP and is in UDMA IV. I am not familiar with Lexar's design, but the one with SanDisk with binary memory (Extreme IV, Industrial, and Ducati) can be used as a HD replacement to a certain extend. The difference between that and their SSD line product is the interface (SATA or CF, which needs adapter to work as PATA), and the wear leveling's robustness for desktop usage profile.
Remember, the performance of Extreme IV's 40MB/S is for FAT file system, NTFS has a lot more writes for integrity reasons and that will not run at 40MB/S. The SSD from SanDisk is of M-System's design and it uses a different flash access engine that is more suitable for windows' main drive.
You got to be very careful on what each brand's promise and reliability. A no name Taiwanese brand may work for 1 week at 300x, but it may not have enough wear leveling to last the warranty period, or it may have only 60x performance in random usage pattern. It is hard to tell without doing some real testing in a controlled lab. If I were you I would buy what Dell is ordering right now, they have very through acceptance test to protect them from bad design and poor quality. From what I understand, Samsung and SanDisk are both approved, but not Lexar yet.