Mobo & CPU upgrade

[ch8ind]

Im looking for a good combo for around $400 that runs well. I game a lot but dont want a crazy system. Id like to play counterstrike-source, doom3, quake4 etc.. im planning just go upgrade my old system and update the motherboard and cpu. i want to use my old power supply, ram, and sound card. i just won a evga 6600 gt so i figured i would upgrade since the card is pci-e. i might upgrade my video card someday but dont plan on running dual cards. i still dont know which mobo i wont to go with but its either a asus or dfi - at most $120... maybe. not sure which exact model.

PROCESSORS
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AMD Opteron 148 Venus 1GHz FSB 1MB L2 Cache Socket 939 Processor - Retail
http://www.newegg.com/Product/Product.asp?Item=N82E16819103596
$229.00

AMD Athlon 64 3700+ 1GHz FSB Socket 939 Processor - Retail
http://www.newegg.com/Product/Product.asp?Item=N82E16819103539
$235.00

MOTHERBOARDS
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Current stats:
CPU: amd athlon xp, 2092 mhz thoroughbred - current clock ratio 12.50x - fsb 167.3 mhz - L1 64KBytes - L2 256KBytes
RAM: corsair DDD-SDRAM 200mhz PC3200 value ram
Motherboard: Asus A7N8X nvidia nforce2 Ultra400
Video Card: ATI sapphire 9600 (ATI RV350) AGP 8X - Processor Clock 324MHZ - Memory Clock 378 MHZ
PSU - ASPIRE ATX-AS500-SV 12V 500W

New stats:
CPU: AMD 3700+ or Opteron 148
RAM: corsair DDD-SDRAM 200mhz PC3200 value ram
Motherboard: Asus or DFI
Video Card: Evga 6600 gt
PSU - ASPIRE ATX-AS500-SV 12V 500W


suggestions or anything else would be nice thanks