|Theres no one main thing that makes your computer fast, there are just lots of different things:|
First one is free - make sure that you take good care of your computer, antyvirus, antyspyware, dephragment etc.
Make sure that your ram is fast and that all the sticks are of the same speed, or else they will all run at the slowest speed.
Always get a dedicated graphics card - doing this preserves ram and is the obvious thing to do if your a gamer.
Physically clean the computer often - ground, open up machine, remove the heatsink and use compressed air to remove dust, reapply thurmal paste and reassemble; this helps prevent instances where the cpu will overheat, perform slower in order for it to cool down and then overheat again etc etc.
This might not be that fitting in your situation, but just fyi, if your into building stuff from old parts - ie: rescuing stuff from skips etc, make sure that you think logically about what should go with what, for example, even though a 10gb drive is just about ample for a xp installation, considdering that most of them are 3600 or 4200 rpm, this makes them a poor choice as the rpm is to slow for xp; I suggest 5400 rpm and above only.
I haven't even touched the tip of the iceburg here - but i'm sure someone will make some additions to what i've said.