Yea they're great, but you'll probably need an adapter to fit in case and if I remember you have an old m'bd check you got sata connections. Whether or not you'll get an increase in performance is doubtful in an old computer, for bang for your buck you'd probably be better with a HDD.
But we're all pulling our hair out as unless you've bought a new set up you're chucking money away Dean, honestly mate you would be better saving up for a new computer, or at least an up to date M'bd/cpu and memory, if you know what you're doing. In fact (I'm going to get some stick for this) from what I've seen of your circumstances the way to go gaming would be a console, then you can just plug and play with no troubles. We would of course disown you, but you can live with that, loads of cheap used consoles around, find a one that plays the games you want, it will be cheaper and better for you.