That 945P Neo3 should be fine as it has one PCI-E 16x slot. While PCI-E 2.0 does provide more performance the real world difference is negligible. The real question is, can your power supply handle a Radeon 4870 or an nVidia 260?
Getting a big good video card like that will just be bottlenecked by older CPU's and getting best of the best for the best price(as mentioned by megamanx00) with that said and the 2.0 pci express slot not there, you wont get maximum results.
But anyhow... Check the back of your case, see if you can see the Power Supply label, somewhere in the middle of the label should say a number ending with a w. For example... 500w, 450w.
For one of those cards id recommend a good brand 500w Power Supply.
there is more to it than wattage , on teh power supply label you should see somethign like this +12 volt this means your +12 volt power rail, a 260 or 4870 will draw about 26 amps off this particular power rail , so you need a power suppy that has it's +12 volt rail rated at a minium of 30 amps (as the CPU also draws it's power from this rail as well ) ... shop at new egg and look at the PSU's specs there they spec out all the rails at newegg so youc an match it to your needs . also keep in mind a single really ehavy duty +12 volt rail is better than 2 smaller +12's as is common with many psu's today , some of the best single +12volt rail PSU are
PC Power and cooling
Corsair
and Seasonic
forget antec and thermaltake as ALL their models used a series of smaller +12's instead of teh mroe efficent single big +12's
also note that NOt all silver stone models are single railed so unless you really look at the model you are ordering you migth get one that has multi +12's
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