No matter how hard I try, I can't get under the $450 budget with a computer that I would honestly recommend to you for what you want. Core 2 Duo's (and Quads) have recently been replaced by Core i5's (and i7's) and they will have very little upgradability. The cheapest i5 (the 750) is $200 which puts it well out of your budget range. AMD has some nice options though, with good upgradability and performance.
You'll also want a good aftermarket cooler if you plan on overclocking which will run you around ~$50.
Cuts can be made here and there, and you actually might be able to bring it in under budget, but I would recommend putting away a little more money before taking the plunge. An extra $200 added to the budget will drastically improve the performance, reliability, lifespan, and upgrade-potential IMO.
I don't know if you are in the United States but if you are (I assume from you using NewEgg.) see if there is a Fry's Electronics near you. I know some people hate them but some of their combo deals are unbeatable. They have had an E7500 and MSI P43 board for $100 about a week ago and they usually repeat specials. If you can get that deal again that would be a great starting point for a gaming build. This week they have and E8400 and the same motherboard for $120.
It shouldn't be to hard to get the other pieces for $350.
If you don't live near a Fry's (or a MicroCenter which also offers good CPU prices as well) maybe consider go AMD so you can get a socket AM3. Which will be more upgradable 2 years from now than a socket 775.
BTW, I checked new egg and saw an ATI 4850 512MB for $99.99.
Suggestions to save money on that original build I posted:
New HDD suggestion ~$20 saved. Good performance at an unbeatable price. Samsung F3
New PSU suggestion ~$65 saved. A decent PSU that isn't too expensive (especially after rebates). OCZ StealthXStream 700W
before MIR: $566
after MIR: $521
If you still need to cut costs, change the CPU. I don't really recommend doing this unless you have to. ~$15 saved. The 545 also has a combo deal with the same motherboard.
--COMBO DEAL-- PII X2 545 Callisto
I'm still $116 over budget, but maybe that new price will be a little easier to swallow.