I had the same question as you recently, and the answer is . . . it depends!
Well, it doesn't depend on very much, actually.
Since you've already got the SB800, it doesn't matter that the SB600 can't be used as a main unit to power a remote unit. You've got the SB800 for that. So the only practical difference (apart from about $100 in price) is the slighly lower power of the SB600.
I added three SB600's to my SB800 so as to take advantage of some of the neat tricks you can do with Nikon's Creative Lighting System. For me, the main one was that, in addition to the SB800 itself, you can control each of the remote units individually and wirelessly from the SB800, setting each one to a different flash EV level.
Haven't tested the range yet, but the manual seems to suggest that the SB800 can control remotes that are within about 30 feet or so.
I got another SB-800 as my second flash. The reason is more flexibility. If you plan to stick to iTTL/CLS the SB-600 is OK. If you want to start using the flashes in manual/semi-manual mode the SB-800 is far more capable because of the built-in SU-4 and the more granular control of the power level in manual mode. The SB-800 also comes with some accessories that you'd have to purchase separately with the SB-600.
I started with one SB-800, then added three more last week.
I really labored over the decision to order 3 SB-800's vs 4 or 5 SB-600's for the same price.
I chose the SB-800 for the following reasons
1., $100 price difference becomes $75 with the Nikon USA rebate program
2. includes $15 diffuser, which is a useful thing
3. I only want to learn one instruction manual, by using all SB-800's it increases consistency
4. I can split up my flashes in any way and still use any one as Master, whether it;s becuase that is the first one I grab out of my bag or because I am using two cameras, each with separate multiple flash set-ups
5. I like the use of a 5th battery to improve recycle times.
Over the long term, as you spend gobs on your camera gear, I think saving $60 here or there is not as important (at least to me) as #3 and 4 on my list.
I'm using a couple of cheapo Velbon tripods for flash stands. I currently have one SB800 and one SB600. At some point, I plan to acquire a second SB800 and then trade the SB600 against a third. The 600 works fine, but remembering two different modes of operation is a pain.
Are you using the D70 in commander mode? If you are, be aware that you can't control lighting ratios via that setup. i.e., all slaves will fire at the same ratio. So if you want to use one as a key, and one as a fill, you'll need to have the key light (master) physically connected to the camera so that you can control the fill lighting amount.