For the cpu.
It all depends on how you'll use it. Will you be the kind to rip/burn dvds frequently while playing games for instance ? If your answer is yes, then you need an X2. AMD had a huge price drops not too long ago so they're very affordable, which one you get all depends on the money you're willing to spend.
Now if you're answer's no, then an athlon64 will suit you just fine, and they're cheaper than the x2 of course.
For the video card.
Since you have an SLI board, which gives you the possibility of having 2 identical cards working as one, improving gaming performance considerably, you should get an nvidia card. I prefer ATI myself but it would be incoherent to get anything else than an nvidia card for our purpose.
Again, which one, and how many of those you get, depends on the money you're willing to spend. Something along the lines of a 7600gt or 6800gs should suit you fine, more than that and the cost grows exponentially. That's not to say pricier cards aren't worth it, because they are.
I would only advise going the SLI route if you're willing to dive in all the way, i.e. getting a 7900/7800, because for pretty much anything less, you can get equal or better performance in a single card.
Also worth, mentioning, the 7950gx2 which has 2 cores, so 2 of those and you get 4 cores :D Needless to say, they cost a premium.
WARNING: As one single card already sucks up a lot of juice, twice as many will consume twice as much, which you means you'd need a very psu, and good psus are pricy too.
I'm sure other people will give you more info as for which card to get.
