To do SLI without a hack (its an easy hack but it still makes people nervous) you need a intel or nividia chip set on the mobo southbridge(?). AMDs chipsets dont allow SLI only crossfire, although im fairly sure nividia chips allow crossfire... odd considering they dont let you use a physix card with a amd card w/o a hack (once again its an easy hack, but its still a hack none-the-less)
If you dont actually plan on going SLI with your 450s, I say just use an AMD chipset with an AMD mobo, as sometimes those nividia chipsets when running on am3 boards freak out and do some odd things, nothing major, just odd.
I actually would say go for the x6 the cost of 2 extra cores is only 50 bucks and even though it wont make much of a difference now (like 4-5 frames), I am sure it will in 2-4 years when games actually start using those cores, and if you use apps like photoshop or video editing, you will notice the difference instantly. However this artical will disagree with me D:
http://www.tomshardware.com/reviews/...i-x4,2791.html