Look at the prices too. 120GB ain't gonna be all that much more expensive than 40, tbh. EVen then, 160 or 200 ain't gonna be much more than a 120. Always better to get as big/fast as you can afford. You never know, REALLY, how much space you'll need. 120GB is a good starting point though.
for you, though, 200GB or 300GB will probably be overkill.
AMD Athlon 1700 (@2200mhz from 1466 sod the noise!) - 768mb pc3200 - 200GB Maxtor - Pioneer 106 - GF4 Ti4200 - twinhan/visionplus pci dvb-t - random pci analogue tv tuner - A7v600-x (to replace the a7n8x-d that died...) - random 17\" CRT
Sony STR-DE425 & Eltax c205s;
xBox w/20GB HDD- executer 2.3b;
xBox - standard;
Konica/Minolta dimage z10 w/2x512mb SD;
A silly amount of Cat-5 and usb cables