Had a bit of a mare logging on, looked like the server was down - but its back up now

As some of you may know I currently have a pc as follows:
Athlon t-bird 1.33
256 ddr 2100 ram
20gb h/d
generic soundcard (which cost 10 quid and is v good!!

nvidia geforce 2 mx 400 (or 440 i forget)
now this system cost me about 700 squid bout a year ago (built meself, but yes it works ok...

Now i want to upgrade- X2, Freelancer, others and existing games I want FASTER!!!!!
So, what I want to replace is:
Athlon xp 2100
512 ddr fast ram
geforce 4200 ti
rest as before, but maybe bigger h/d
so...my question is should I upgrade (cost about £230) or should I build a new pc (which would be fun - yes I am that sad11

big question really is how much could I get for my present pc (not incl monitor and speakers, keyboard, mouse)??????
Thanks for any advice
Pixel