Corvette isn't online on XBL or PS2. Ford Racing 2 is online on XBL, but only with scoreboards - no actual online racing.
I had gotten NFS:HP2 for $16 and change as soon as it went Greatest Hits, and it's a much better and deeper game than Corvette or Ford. From the reviews I've read, Ford gets the job done but is pretty boring, and Corvette feels like Need for Speed but that you have to seriously love Corvettes to choose that game over a real Need for Speed game.