Switching to SLR-need advice

[EOS Novice]

I've been using a Canon Power Shot Pro 90 is for a couple of years now and really like the convenience of digital photography. I hate to carry a bunch of gear around and appreciated the range my 37-370 lens (35 mm equivalent) gave me, but I've finally came up against the lack of mega-pixels. I just can't make a decent sized print with what I have. I am strongly leaning to the EOS 30D, but am unsure of what lens to buy. The 17-85 mm looks like it might work for my walking around needs, but I have recently started shooting auto races. Any suggestions on a 300mm telephoto? Is the quality of 3rd party lenses like Sigma or Tamron any good and do they autofocus? Any thoughts from experienced users would be appreciated, thanks.

[longroad]

If megapixels are your only concern then a DSLR is overkill.
Most people buy a slr because they have come to a limitation with what they can do with their regular camera, usually in the shape of not enough zoom, aperture, ISO, shutter speed ranges and general flexibility with what you can do with the camera.

A Panasonic Lumix FZ50 has 10MP, same as most mid range DSLR's and without the extra baggage and cost that comes with SLR lenses.

[coldrain]

The best 70-300 lens you can get is the Canon EF 70-300 f4-5.6 IS USM. It is more expensive than the Sigma and Tamron 70-300 lenses I think you are hinting at, but that is worth it definitely.

From the cheap 70-300's teh Sigma 70-300 f4-5.6 APO DG Macro is the only one to consider, in my opinion, since all the rest have a big slump in resolution and contrast towards 300mm.

But the Canon 70-300 IS USM is very sharp, upto 300mm, and sharp into the corners. It has virtually no CA problems, it is contrasty and has good colour, and it will focus faster and more accurately... all in all this is the lens you should aim for if you do not want to stretch your budget to a very unaffordable level.