Re: a little updrage confliction.
Yeh that mobo you have is still pretty good, the P45 is not that much better (overclocks a bit more etc). As ferarri said, getting a 4870 would be better, cheaper and easier. It should be fine in an Antec 900.
The PSU you have is fine for either a HD4870 or 2x 9600GTs.
Also, SLI 9600GTs will not be as good one 4870 i dont think. It may come close, but considering the issues with SLI, it is easier and more efficient to stick with one card.
If you do decide to go with a new mobo, i recommend nothing less than a full re-install. This means; a format or a clean drive, a reinstall of windows and all your drivers and programs. You will hear of people managing to do it with swapping the mobos and drivers or with a windows repair, but i would strongly advise against this. There are to many possibilities for conflicts otherwise (im pretty sure most here would agree).
EDIT: Bah, satlynay got in before me...