Why would you want to use 'cheap ddr' on expensive servers? As in cheap, do you mean non-ECC registered or do you mean one of those cheap generic brands found on pricewatch.com that carry little compatability?
If your talking about regular ram, and not ECC, then I suggest not going the cheap way. The ram you see for really cheap, was weird module configuration, that works with only a few chip sets.
At this point ram is so cheap, you can spend $20 more and a good brand, with great compatibility. www.newegg.com has been having ram specials the last few weeks. 1 gigs of ram in 2x512 or 1x1024 for around $80 from corsair and Kingston
Well, that's because the larger the member module, the more useful ECC will be since as the module gets bigger, more room is left for error so the Error Correcting Code (ECC) will benefit you. On the ECC RAM, there are extra bits that are used to correct the data passing through.
There are are several tyan boards with 8x ddr slots(for amd and intel). I havent used either of the boards I found, so Im not sure if they do ddr, and ecc ddr or not. But you can get muskin 2gig(2x1024)sticks of ram for $250 a peice. or a single 2 gig stick is like $440
Opteron design has ECC everything inside the chip for reliaility. If you want to use non-ecc ram, you may as well just use athlon64, I think athlon64 x2 4200 is perfect for web server. inexpensive and ultra performance.