Bec, I would recommend StatsRemote.com - it's $29.95 per month, but it's well worth your money. It makes it so much easier to track sales and see how well you do with certain sponsors.

I have tried both Nifty and StatsRemote and I think SR is the best there is. Their support is awesome too.

As for the links to sponsors, it's probably best to use a jump script (either one per domain or one for all your sites combined). It enables you to change your outgoing links in an instant.

I can send you a sample of a single php file that does this. Let me know if you (or anyone else) need(s) it.