Yes, if you get a 2.0 card, it will work in a previous revision PCI-e slot. I would personally, to save space and heat, get a one card solution, of course, for me, I only have a 22 inch monitor and my one 8600GT is more than enough for most games, so yeah, my eyes can't tell the difference if my game is running at 60 or 100 fps.
I am not good with motherboards, like I just bought this Biostar because it's reliable, but I didn't know it can't OC over 7% (That's weak sauce right there) when all my Gigabytes could get the proc well over 15% no problems, so I'm looking for an upgrade myself, I'm going with a DFI board in the Infinity series next personally. I can only assume that your Pentium D is a socket LGA775, this board is on the cheaper side, but for the price, I think the features you get with it are quite good, here's a link.
Newegg.com - DFI INFINITY P965-S LGA 775 Intel P965 Express ATX Intel Motherboard - Retail
Of course, there are more expensive models in the Infinity line, but this is the one that has the better BIOS for current and future procs.