some basic new computer questions

Neither SLI nor X2 are worth it from a price/performance ratio standpoint, but if you want top of the line and have money to waste go for both.

If you're interested in building a good system for a decent amount of money, go for neither.

NewEgg is a good place in general, but if you want to save money, check out each part on AnandTech RealTime Pricing Engine.

I'm sorry but I'm going to have to completely disagree with Extrarius on this one. I have seen computers running duel SLI GeForce 7800's and would say (from a personal standpoint) that it is well worth it. Not only do I say this because the performance is ridiculous by today's standards, but 2 and 3 years down the line a system like this is going to still be hanging with the top of the line software out there. It's a worthy investment if you ask me. Also, I have an 64bit AMD athlon and have been extremely pleased with its performance. My friend got a P4 64 bit at around the same time I got my AMD (about a year and a half ago) and his has long since burned out...not saying this is always going to happen or that it's even common at all but from what I've seen and based on a number of benchmarking, AMD is the way to go.

The new Athlon 64 (FX, X2, or otherwise) blows away all of the new Pentiums. They are also cheaper. At least, in 99% of the benchmarks I've seen they have.