Once you hit 4 gigs of ram, you're set. Tighter timings will help a little, but I wouldn't spend too much more on them than a standard set. You'll see better returns from your GPU upgrade. The P35 chipset only sports PCIe x16 1.1. So I wouldn't consider crossfiring 4870s on that board. x8 on 1.1 is too low for that card. If you are keeping the P35 board, the 4870 1gb is a great single card solution. But if you're playing at 1680x1050 or lower, the 4850 1gb would do well for less money. The C2D should OC nicely though. Might be worth buying the GPU first and OC with the stock fan, seeing how the results are. If you like them, OC with an aftermarket fan and upgrade the ram. If you want more, go for the gusto and fully upgrade. You'll already have the GPU.
If you're doing a full guts upgrade with ram/cpu/mobo/ and GPU, look at AMD Phenom II x3s. They are one of the best price/performance chips right now. If you want CF as an option, the 790GX is nice and the 790FX cost more but will have a little better CF performance. If you're only doing a single GPU and don't want to CF, then a 770 or 790GX is a good option. I'd get the DDR3 boards and ram though. Its a little more now, but if the AM4s (many many moons away) supports the next gen memory standard and DDR3 (like the AM3s support DDR2 and DDR3) then you won't have to buy new ram on the next system upgrade.