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.
I am working on a gaming rig, and I have been out of the PC hardware circuit for awhile. I have had my current system for 3-ish years. The motherboard has never been what I needed (and it has so many incompatibilities with memory that I've got beautiful OCZ Gold and Corsair DDR2 DIMMs that have never seen use just laying around). I also bought a single-core processor thinking, in my infinite wisdom, that dual-core was a gimmick -- WAY wrong, I know, but I am stubborn.
I am currently running an EVGA GeForce 8800 GTS 640MB DDR3 on my system right now, which one would think is a mighty powerful card, but my system still chugs when trying to play a modern game, presumably due to the lack of quality memory and a single-core processor.
My first question: Are my assumptions here right so far? Is the processor the limiting factor for why my PC chugs in modern video games (e.g. Fallout 3 at 15 fps, BioShock at 20 fps, and even Darwinia which is at an un-runnable < 1 FPS).
I have been looking at reviews and write-ups on the GTS 250, and apparently it is, more or less, a slightly-souped-up 8800 GTS that uses a good deal less power and has the newer PCIe 2.0 16x interface with its greatly-improved bus speed. I have also been considering using my 8800 GTS in the new rig, since PCIe 16x is backwards-compatible with 2.0.
My second question is: Would you say that the GTS 250 is a worthwhile upgrade from an 8800 GTS?
I have also been intending to use my DDR2 DIMMs from OCZ Gold and Corsair that I have left over from my current system that didn't like them.
My third question is: Is DDR3 really that much better, considering a lot of motherboards still support DDR2?
And finally, do you really think DirectX 10 is worth a $200 OS upgrade from XP Pro x64? I really don't want to go to Windows 7; there's so much about that OS that I don't like, and I am not just a gamer; in fact gaming is an important-but-secondary function of my PC.
Here's the specs I'm looking at for my new system:
CPU: AMD Phenom II X2 550 Callisto 3.1GHz Socket AM3 Dual-Core
MB: MSI 770T-C45 AM3/AM2+/AM2 AMD 770 North Gate, 4 DDR2 DIMM slots, PCIe 2.0 16x, no onboard video (big selling point for me)
Video Card: EVGA GeForce 8800 GTS 640MB/DDR3 PCIe 16x
Power Supply: Rosewill RV2-500 500w ATX 12v v2.2 EPS12v
Hard Drive: Hitatchi Deskstar HD31000 IDK/7K 1TB 7200 RPM 32mb cache SATA 3.0Gb/s 3.5"
+ 1 DVD+/-RW Drive
Case: Rosewill Smart One ATX Mid Tower Black w/3 fans preinstalled