1| Along with that $300 i'd sell the rig and get a totally new build from us
2| With the PSU, CPU and a low profile card - your overshooting your luck with a gaming rig by a mile but you'll manage to pull it off just that your not anywhere near the targeted gaming performance of an equally priced rig
add a seasonic PSU to it, http://www.newegg.com/Product/Product.aspx?Item=N82E168... - now comes the CPU - that'd be a lil hard as we don;t know your mobo and IF it can support a 45nm Intel Quad core, ok i've overshot it but it will support 65nm processors - still looking at your rigs documentation. oh and add another 2GB DDR2 (800-1066MHz) of ram (if your slots are free that support 1066MHz) and it comes to more than $300
feeling ppoped out with all this stuff to look at and find - 530a/b/c/d but no bloody freakin 'S' lols...when did dell ever keep up with their aging hardware? on another note, HP and dell are very alike, they have propietory parts and the first problem people talk about is the PSU swap. But i think that trouble has passed away.