Which CPU and GFX card for upgrade?

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Am toying with an upgrade, especially as I got given GTA4 for crimbo - probably just GFX card and CPU. I'll then donate them to my partner's son, as I bought him a mobo for xmas.

Just wondering which quad chip to go for, that my mobo will like. And which are better atm - Nvidia or ATi? Toying with either GTX 280 or HD 4870.

Current System specs:
Gigabyte GA_965P_DS3P (Socket 775) PCI-Express DDR2 Motherboard
Intel Core 2 Duo E6420 "LGA775 Conroe" 2.13GHz (1066FSB)
Crucial Ballistix 2GB (2x1GB) DDR2 PC2-5300C3 667MHz Dual Channel Kit
Geforce 8800 GTS 320MB
550W PSU

Cheers

 

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Any thoughts on these:?

Intel Core 2 Quad Q9550 LGA775 'Yorkfield' 2.83GHz 12MB-cache (1333FSB) Processor

OcUK Extreme GeForce GTX 280 1024MB GDDR3 TV-Out/Dual DVI (PCI-Express) (when it comes...)
or
Powercolor ATI Radeon HD 4870 1024MB GDDR5 TV-Out/Dual DVI/HDMI (PCI-Express)

 

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Cheers for that lads - think I'll hold out for the GTX 280, and stick with the current CPU, which is clocked at 3.0Ghz atm. Will probably chuck in another 1Gb of RAM - tis cheap enough.

Don't really want to replace the mobo yet as I tend end up replacing everything when I do!