We (my former employer spit) tested MS NLB out in the lab, and ran into a couple of problems. The biggest was the amount of broadcast traffic generated by NLB, even in a test environment.
If you are going to use NLB, I would recommend segmenting that part of the network off with a router (which kind of defeats the purpose of redundancy...but it is better than taking down the whole network with a broadcast storm
I'll preface this by saying I am a hardware bigot, I prefer to see things done with hardware rather than software, but I agree with apollo, you are better off with a hardware solution. Foundry ServerIrons are only $8K or so a piece (more with discount, and Foundry is desperate for sales right now). In my opinion you get better redundancy, and more efficiency with hardware over software.
If the $8k price tag is too high, Coyote Point (http://www.coyotepoint.com/
) makes decent boxes for about half the price. I wouldn't use them to load balance multiple websites, but they can definitely manage a single high traffic site fine.