It depends on what kind of track you're going to on what kind of tires you should buy.
If you're going autocross/road racing, there are really really really good tires that have DOT ratings so they CAN be driven on the street (even though it's a retarded idea!). However, these tires are complete gumballs and they have a treadwear rating of 40 (I bought a pair of Nitto 450 Z-rated tires with a treadwear rating of 300, for reference), but they're sticky as all hell. You'd probably need new rims to mount them on as well, because using them for anything other than the track would be a waste of money. They're expensive, too; $221 a corner for Michelin Pilot Sport Cup tires.
Linky:
http://www.tirerack.com/tires/tires....ilot+Sport+Cup
Other competition tires include:
Dunlop SP Super Sport Race, Yokohama Advan A032R, Pirelli P Zero Corsa, and BF Goodrich g-Force T/A Drag Radials.
I haven't used them personally, and you have to heat them up before they work right, but they'll probably help your lap times. The stock SP9000 Dunlops or whatever are reasonably good tires for their price & the fact that they're original equipment.
Good luck; didn't see where you're from, but maybe I'll see you at the Auto-X this summer!