Buy an old Gixer and grin like this!
http://www.stopstart.fsnet.co.uk/newbuttons/biggrin.gif

There's something to be said for a lightwight kit car. Have been round a track in an old sylva striker with bog standard 1.6 crossflow, with a well balanced chassis still gave silly corner speeds and a whole lot of fun for not much cash.

If you want more speed, spend a few k and get something with less weight and more power. My brother has a Fisher Fury about 550kg, with 180bhp Zetec. 0-60 in v. low 4s, grip at caterham level, took seconds a lap out of any Elise we could find. IIRC, cost him something like £8k to build, might get £6k if he sold it now. Good on sunny weekends, and the kit scene is even friendlier than the scooby scene.

Now all I need to do is learn to drive the things properly ;)

Find someone who knows the cars and you can pick up some rapid and fun machinery second hand for sensible money.