4650 isn't a great card tbh, I'm a massive hardware junkie. The 9600 is superior and that's a few years old now.
However. PC world's PC aren't really that great, even the gaming one screw 2 shoddy cards together and pray for the best. What you need in your PC to run SupCom 2 is a good processor, it deals with Physics and AI. That's pretty much all SupCom 2 is, Physics for the bullets, AI to point the bullets at you. A quad core will be better in this case than a dual core. Not often you get to say that. That 4650 won't let you max out your settings but that 9600GT with a bit of an OC might just do it.
What you're looking for is: Quad core, 4Gb RAM, decent GPU. It's hard to get a full PC for £500 that can game, especially store bought. What I suggest you do is buy a store bought PC from PC world, a quad core with 3-4Gb RAM and whack in a shiny new GPU. PC World's PC's will support a GPU on their PSU's so you've got nothing to worry about.
Buy a Quad core PC, whack in an ATi-HD4890 and you'll eat through SupCom 2 no problems