This may be too rich for your blood, but I am selling a few items on photography-on-the.net/forums.
1D MarkII - $825 for local transation
16-35 f/2.8L - $825 for local transaction
I'm am also selling my Lowepro Slingshot 200AW bag for around $40 if ur interested.
But to be honest, I don't like that bag much. Like c2000 stated, for short trips, it's ok, but when you're lugging around gear on one shoulder all day, it isn't comfortable. Depending on how much gear you have/want to get, I'd suggest the crumpler 5MDH shoulder bag or if you have more gear, the flipside 300. I'm using the flipside right now, and it's one helluva bag.
I don't know what your budget is, so it's hard to recommend things to look out for, but I think you are along the right path with a 50D Tamron 17-50 f/2.8 combo. The AF is fast and accurate (although a bit loud, if you care about that) and the IQ it provides is very good. It's the best bang for your buck you can have on a crop body.
Now if you want a telephoto to shoot things at track days and such, I'd suggest getting the 70-200F4L. The Sigma f/2.8 is good, but the AF isn't fast enough to track fast things. The EF 70-200F4L is about $500 used, super sharp images, colors and contrast are good, and best of all, the AF is amazingly fast and accurate. Plus, track days are usually during the day so you have more than enough available light to work with the f/4. You can choose the IS ($900 used), f/2.8 ($950 used), f/2.8 IS ($1.6k used), or f2.8 IS mk2 ($2.5k) versions, but it's obviously going to cost you more.
If you want deals, I'd join the POTN (photography on the net) forums. I've bought & sold close to $3k's worth on there and never ran into a problem. Great community to learn about photography, too!
btw, my gear list:
1D MarkII (for sale)
16-35L (for sale)
Sigma 50mm f/1.4