Also, unless you are running a 64bit operating system, your 4GB of ram will only show up at about 3.3GB at best.

I personally ran 4 in one of my systems for a while, but didn't really see that much of a boost over the 2GB mark, at least without changing to a 64bit op system that can properly utilize your hardware

If you have 2x1GB and 2x512MB sticks, then you can configure to run at 3GB, which is the max for a standard x86 op system, otherwise I would suggest just sticking to 2GB.

The upgraded mobo and proc look good (although I personally loath Intel processors, but hey, I've only blown three of them up :P ) They should do you good, I have a couple friends that swear by the Core2 series.

Now, if you want peek performance, and know how to run a linux system, Ubuntu x64 would be the way to go, then XP Pro x64, then finally, Vista Ultimate x64. Vista is nice and pretty and all, but the overhead on the op system is horrid compared to Linux, or even XP.

I run XP primarily because it supports Vanguard, which is not currently supported by Whine or Cedega, at least not the last time I checked. Other than that, I would be running Linux on all my systems.