In that case, I would get something like this for a drop-in upgrade:
Part list permalink /
Part price breakdown by merchant /
BenchmarksCPU: Intel Core i5-2500K 3.3GHz Quad-Core Processor ($179.99 @ Microcenter)
CPU Cooler: Cooler Master Hyper 212 Plus 76.8 CFM Sleeve Bearing CPU Cooler ($22.99 @ NCIX US)
Motherboard: Biostar TZ77B ATX LGA1155 Motherboard ($97.55 @ Newegg)
Memory: Patriot Signature 8GB (2 x 4GB) DDR3-1600 Memory ($41.99 @ Newegg)
Power Supply: SeaSonic 520W ATX12V / EPS12V Power Supply ($65.98 @ Newegg)
Total: $408.50
(Prices include shipping and discounts when available.)(Generated 2012-04-19 10:57 EDT-0400)I am starting to like the Biostar T-series good price/performance, cpu voltage regulation just can't be beat at that price range. Used 2 of them recently in 1156 builds, very decent. Can't go wrong with the Seasonic 520 it's solid and can handle a GPU if you see fit in the future (the 7850 would make it a really nice game rig

). Oh, and your ddr2 in teh p5nd won't go so I added some ddr3 for you.
Speaking of the P5ND, perhaps just grab the PSU for now, and OC that sucka you should be able to hit 3.4 ~ 3.6Ghz just by futzing with your clocks (depending on your memory speed and multi selections, of course) probably what I would try just to get some more life out of it. I currently have an E3400 @ 3.4Ghz on a P43 board the stock is 2.6G same as yours

I have ddr2-1066 running @ 1:1, probably could go higher messing with it but it's super-stable (my linux workstation yay)
cheers