[Fyron]

Vista does not have any native DX8 code; all the earlier DX stuff gets emulated in the new layers. This results in typically poor performance. Most DX8 games were designed in a bygone era, and do not stress modern hardware at all, so its not as noticeable. But with SE5, the game engine was made with modern (if lower end) machines in mind. Performance in XP is ok, but the translation layer in Vista hurts it quite a bit.

I should also point out that you should not expect "Quadcore" to make a lick of difference for 99.999% of games, since they tend to be single-threaded processes. You end up with lower performance compared to a dual core CPU with individually faster cores (there aren't any modern high-end single core CPU designs, but such would theoretically perform even better). 4 GB RAM is way overkill as well, since no process in XP can normally use more than even 2 GB of (physical) RAM, and they are designed with that in mind (ie: there probably are no games yet that will use more than 2GB even in Vista/XP 64-bit OSes).