Your results are pretty much why gamers are never happy for long even if they build a decent system. A new game will place even greater demands than what the old system can deliever or you hit a wall. The plain fact is, "gaming" expects the most you can ever deliver or build. Effectively, sooner or later you'll be lacking or worse it starts to breakdown all too quickly.

Whatever, you build make it the best you can "afford" and if possible wait and save $ to buy what is needed. I mentioned this, if the game requirements are such and you can get them but raise the level abit, you'll be happier longer unitl an upgrade or new system. Multiple core is better but again, buy one that suits your mtrbd., etc., because you can possibility upgrade later. You mention, frame rates, but these can vary against the system you build. I only offer, build the very best you can, because I'm sure you'll be asking what you can do later to improve it. As for DDR2 vs. DDR3, buy the latest as it will be better, though it won't be always noticeable, DDR3 is better. Of course, even amongst brands, buy the premium in order to maintain a level of better play. No, don't mix DDR2 or DDR3 sets, stick with one of the other, of course, the mtrbd. will decide that, just get a DDR3 based one.

On top of all above, "COOL THE SYSTEM AS BEST AS POSSIBLE". Keeping the system cool will allow a better overall performance of componets and lessen breakdown. Add to this that power requirements are well stable, so get a decent namebrand and stable rated wattage PSU. Do all this apparent simple stuff and review game specs, etc., and match and provide even better components will make a good system.

tada -----Willy Happy