compuwar, is there a reason you suggested newer flashes over older, cheaper flashes? I would think adding a few sb-24s and some ebay radio slaves or pc-sync cords would be a better idea. Take a look at strobist.blogspot.com for more info. The added bonus is that your lighting setup becomes ultra-portable compared to regular studio strobes with power packs.
Absolutely- the Nikon Creative Lighting System allows you to basically have the camera do all the "magic" stuff. The CLS is well-worth the money as you get used to the learning curve of multiple flashes. Newer flashes have great recycle times as well. Since the OP already has an SB600, with the addition of a single SB800, they'll have wireless flash sync and *control* automagically. They'll still be in the ultra-portable category (if you look, you'll see that I recommended they check out Strobist)
If you're going to be a shooting pro, you're going to end up with PWs, no matter what, but then you're going to have to get the flash settings right no matter what. With the CLS, you get TTL flash to multiple flash heads within IR line-of-site and you can control the settings of each strobe from the SB800. With the 800 dialed down for fill, and the 600 set as the key off and up to one side, you've got a perfect 2-light system that just needs a bracket for the fill and a stand for the key without worrying about cords, dancing back and forth for settings, etc. It's a great way to learn while making easy adjustments at the camera.
I'm not saying the SB24 route sucks, but if you've already got a start on the CLS, why not step up. If the OP upgrades to a D200 in the future, then they'll have a 3 light system right there (I'm pretty sure the D200's flash will also act as a light in commander mode, which you don't get with the D70s, which will act as a commander- giving you the option of using SB600's instead of 800's.)