Crossfire or just a new video card?

Hey all!

I'm building a gaming computer (my first) and I have essentially what I want mapped out. The parts look like this:

ASUS AMD 790GX/790FX Motherboard
AMD Phenom II X4 940 Deneb 3.0GHz Processor
ASUS Radeon HD 4870 512MB Video Card (with Crossfire support)
OCZ Reaper 4GB (2 x 2GB) DDR2 1066 RAM

First, I want people's opinions on the system. Is there any grossly over- or under-powered part compared with the rest? I think I evened it out pretty well, but like I said, it's my first build.

Anyways, the actual question is this:

I can get one of 2 motherboards:
790GX or 790FX. As far as I can tell, the only different between the two (other than $70) is that the FX can support crossfired dual and even triple x16 video cards, whereas the GX can support only 1. My video card is x16 and I don't plan on replacing it particularly soon. So: would it be worth the extra $70 to buy the FX and then just plop another x16 video card into the motherboard once I need to, or would I be better off just getting the GX and replacing the video card with an upgraded version later? Multiple opinions on the matter would be nice!!

Thanks for the help!!

~~caggles

Unless you plan to game at resolutions beyond 1920x1080, there is no need for crossfired 4870's. It wouldnt hurt to go with the 1 gig version if you want a little more video power for games. 700 is more than enough if you buy a good name brand psu. Most people like pcp&c, corsair, antec. Just do a little checking to make your power supply gives the required 12v amps and connectors the video specs says it needs.

I was originally planning on getting the 800mhz RAM, but this RAM is only $20 more, so I figure I might as well

Thanks for the advice everyone. I'm pretty sure I'll be going with the GX, and then take that money I saved to get a 1gb video card instead.

You guys are awesome!! Thanks so much!!