Desktop Config- CrossFire worth it?

I'm trying to make a gaming computer that will last me for a few years without upgrades, then for a few more with some upgrades. So far I have:

Case: Antec Three Hundred Black Steel ATX Mid Tower Computer Case
DVD: LITE-ON 24X DVD Writer Black SATA Model iHAS424-98 LightScribe Support
CPU: Athlon II 640
Power: OCZ ModXStream Pro OCZ700MXSP 700W
OS: Windows 7 Home Premium 64-bit
Memory: OCZ Special Ops Edition 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600
Video Card: HIS Radeon 5770
Hard Drive: TBD, I'll just find the cheapest HD that gives me 500 GB or more
Motherboard: TBD (read rest of post)

Right now, I'm looking at an ASUS M4A79XTD EVO AM3 AMD 790X ATX AMD Motherboard. Basically, I have 2 questions:

1) Because I want this system to last and be upgradeable, I want a motherboard and power supply that can support an additional Radeon 5770 in CrossFire mode. Once the single 5770 is no longer enough for my gaming needs (probably a few years, since I don't need absolute max settings), I figure I can buy another 5770 and just pop it in. Is this worth it? It's probably an extra $50-$75 for this ability, since I need a bigger PSU and a better motherboard, let alone the eventual 2nd 5770.

2) Will that plan work with this motherboard? It says CrossFire compatible, but I noticed it only has 1 PCIe x16 slot, and I think each 5770 needs x16, but am not sure as I'm somewhat of a novice in this field. Is there a better motherboard for this? And if it's not worth it, which motherboard and power supply should I go with? Do my other parts make sense as well?

Thanks in advance for any help provided.