DIRT 2 can be had for 8 quid at Play.com. I was playing Test Drive Unlimited 1 for a large part of yesterday, that's quite good but not rally or off-road, it involves driving round Hawaii.
Test Drive Unlimited 2 is out now, costs £25 but I don't know if it's any good or not.
Need For Speed Hot Pursuit is a whole lot of fun for nine quid, I paid 30 pounds for that on Xbox. It's not very realistic though if that's your thing.
And of course there's the latest Need For Speed Shift 2: Unleashed Still full price but meant to be a goodie.
And if you're going to be playing racing games at the very least get yourself One Of These