OK, let me hop in here.
First, I would say a dual core, especially that one, should be perfect. A quad core at this point I feel is overkill unless you are going to be trying to run photoshop, Hammer, TF2 all at the same time.
Most games, and most programs even, cant properly utilize two cores at this point, let alone 4. The only game that I know of at this point that does a good job utilizing all your cores is Call of Duty 4, and with my lowly 1.86 Ghz Core 2 Duo, it only uses 65% of my processor.
The reason this is (as I see it), is because it your computer assigns certain threads to certain processors. (sorta) so if the game / program runs in a single thread, it will only use that one process. What game developers / programmers are starting to do is for intensive tasks splitting off to multiple processes / threads. This way you can utilize all the cores on a computer. The problem is then you have to worry about how to transfer data between those threads / processes which is not a trivial task.
If you are going to be doing a lot of stuff other than gaming on this computer (My cousin for instance does a lot of video stuff, so he was able to run I think 8 video transcoding programs in the background re-encoding movie files before his CPU maxed out)
Also, the cheapest Core2Quad is around $270 for 2.66 ghz, while you can get a Core2Duo @ ~3Ghz for less.
As for the motherboard, I have had a pretty bad experience with Gigabyte. I think I have a P35-SP3, similar to that board and it has little to no overclockability and if I turn on the built in overclocking settings the board hard locks up (yes the board, it refuses to boot even to the bios screen), and i need to reset the CMOS. While it could just be Igot a faulting product, I dont think my next mobo is going to be a Gigabyte.
Next point.
4 Gigs of memory should be awesome, if not overkill at this point. I have 3 Gigs and it is glorious. One thing you should know is that if you want XP to actually use all 4 gigs of memory you need to specifically tell it to not use your hard drive as a Paging File (allocate the Hard Drive space for Paging File to 0 mb's). If you dont do that it will only show you as using 2 Gigs of your RAM.
I have heard some debate over whether it actually only uses 2 Gigs of your RAM, or if it just only shows you useing 2 Gigs of RAM, but I figured it was safer to just turn off the Hard drive based paging file since it all fits in my memory anyway.
The video card is perfect, dont change it.
If you do change the PSU stick with sleeved cables, I didnt and it has pissed me off to this day.