Post your system specs and I can give you a rough idea on how GTA IV will run on your machine. At £5, it's worth buying (even if your current machine can't play it too well - it will still be there for you to play on your next rig)
I have the retail version, which is two dual layers in size (so around 14GB).
I assume that the download will be 13 to 15GB.
I'd say you could play the game on a bland of medium and low settings at a "decent" screen res.
This is a "guestimate" though, as GTA IV is poor console port and thus uses far to much resources for what it does.
The game appears to use more CPU resources than GPU, so even my Q6600 with GTX 285 lags on maxed out settings.
Your system should be able to play it lag free though (might need to have primarily low graphic details applied - or small screen res).
As I said before though, the game's a good investment for the future too... I'd be suprised if you find GTA IV for £5 anywhere else.
It really depends on what genre of games you prefer.
If you're like me and love your FPS, you'd love the following games:
-Shattered Horizon (It's MAD on system resources though)
-Unreal Tournament III
-Team Fortress 2
And of course, games such as Batman: Arkham Asylum (which is one of very few games I'd happily play again), DiRT, GRID and even "The Maw" which are great fun.