If you have 4g ram, and 1g video memory on a 32 bit OS, you're not getting your full potential. Install that Vista x64 you got and you should see an improvement.

It sounds like you already have 2 Seagate 750's. While a raid0 of these would give better speeds than a single drive, you will have a 1.5tb drive that you really only need 20gb of for WoW. using your existing harddrives, I'd use one for OS/apps, and the other for your 5xWoW+symlinked data directory. This should also give you a good improvement.

If you want to buy a single Raptor (or SSD) I'd use it for the WoW installs only. This should give you a large improvement. (this is what I did)

GET MORE RAM (nuff said)