Upgrading a Dimension 8300

Somebody had a post about upgrading a Dimension 8250 a few days ago. I'm wondering about my 8300. I have a Geforce FX 5200 video card. I would like a Geforce 7800GS, but don't want my computer to be too hot or the CPU to be bottlenecked and I don't want to spend $xxx on a computer thats 3 or 4 years old. I am wondering about either the Geforce 7600GT AGP 8x card or the Radeon HD 2600 Pro AGP card. Which do people recommend?