One of the things that slow down is teh items you specfied above, the harddrive clogging up and memory. Not sure if its worthwhile to fork out some extra cash onto another harddrive as ur current is quite small for the heavy new applications. As for the memory it woudl be a good investment although you should take into account the other components you have, i.e. processor, motherboard, etc as they could be causing issues to.
Try disabling a few startup applications, delete internet temp files, there was a good pc advisor mag last year that provided help on cleaning up and improving your pc's performance. Or I tink it was last year