You can get an E6600 and a P5B-E or DS3 for around 470 or so and that would be your budget but see...you would NOT see much improvment in games as you do understand that in no way are X2's much slower than Core 2's right?
Your CPU is a very good one for gaming and you really shouldn't worry about upgrading it until the new Agena comes out in a few months.
For now if you want to see really high FPS then get an 8800GTS 640mb for around 350 bucks. Your CPU is a good one and not slow at all.
E6300 w/ Arctic Freezer Pro 7 @ 3.6ghz 1.4Vcore
Antec 900 Case
EVGA 750i FTW Mobo
4x1GB of G.skill DDR2-800
2 8800GT in SLI
22" Samsung Syncmaster
1 250GB WD and 1 Seagate 7200.10 in RAID 0
Corsair 520HX 3DMark06 Score: 13900