Dell :: Updating My Beloved XPS

I'm a bit embarassed to bring this post to the forum as looking down the list nobody's posting about these lappies these days!

Basically, my hard drive's full of stuff I don't want to delete (or move to external), my dvd writer needs a paperweight to keep it closed up and the keyboard is like it has glue under the keys!

So, I can't afford to replace it at the moment and I still love the screen and the occasional Doom3 etc (sad I know). I've done some research here - but would appreciate some guidance on which bits to buy.

The drive - I know it's got to be PATA(?) but which drive should I aim for if any? Also, will all the PATA 2.5" drives fit, do I take the bits off the existing one to put on the new? Is there a limitation on the amount of hard drive memory the system will recognise?