If you don't play games much then not really no (depending what else you use your PC for) and in Windows you won't notice a lot unless you do a lot of multitasking/working with large apps or use programs like Photoshop.
Some games (like BF2) are much more playable with 2GB of RAM and others you can get away with 1GB (especially at low settings).
Adding more memory can unfortunately be problematic, especially with Athlon 64 /X2 rigs cos they hate mismatched memory and to a lesser degree with LGA775 rigs.
If you get the exact same model it doesn't necessarily mean they will actually be identical and compatible cos they change slightly all the time, using them in seperate dual channel pairs will improve your chances but it can't be guaranteed.
It is best to sell your current RAM and buy 2x1GB kits if you can get a decent price rather than risk it and be stuck with 4x512MB modules and if your buying new I'd stretch your budget unless it really is only an office/casual gaming machine.
