It might be more price beneficial to purchases a p45 motherboard and than a 4870x2 if you plan on crossfiring anyway. That would knock a $100 or more off the mobo cost and save $50 off the gpu total assuming prices are about the same when you purchase your next card. Otherwise I'd go with a 1gb version of the 4870. Also, low latency ddr2-800 might be worth the price saving depending on what your hoping to attain in overclocking. I have geil 2x2 gb ddr2 800 and at 2.1v i'm at 1053 Mhz and 5-5-5-15 timings and would recommend it unless your really going to be pushing the memory. I haven't played around with the timing either to see if I can tighten them.
If you get a 4870x2, you only use one video card so you do not need a crossfire motherboard with two 16 lanes pci-e connector, so a P45 mb is enough. If you want to get one 4870 and maybe another 4870 later... you should get a motherboard with two full 16 lanes pci-e connector (x48).