If you want to play Doom 3 get a graphics card upgrade. You won't need that fast a CPU, since Doom 3 will be mostly GPU-bound. A game like UT 2003 on the other hand is mostly CPU-bound. My Radeon 9000 Pro is hardly faster in it than my GF2, due to my slow 1 ghz CPU.
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CPU: Intel Core 2 Quad Q9450 Mobo: Intel DX48BT2 Memory: 8192 MB PC10600 DDR3 Videocard: PNY Geforce 9800 GX2 Soundcard: On-board SigmaTel High Definition Audio Hard drive: 1 TB Seagate & 1 TB Hitachi Optical drive: LG GGW-H20L (2x BD-R DL) PSU: Nexus 1000 Watt OS: Microsoft Windows 7 Ultimate (64-bit)