for CAD of any sort AMD always seem to perform better. and ATI is more of a game focused card, Nvidia performs a bit better in the 3D mark, try getting a Geforce ti500 and get the softquadro hacked drivers to make it run better in CAD.
if you can afford it go for somehtign between a AMD XP 2200+ - 2600+ thoroughbread. (from 2400+ and up its getting harder to find them).
or you can wiate for the AMD Hammer cores that will wreck havoc in everything in its path of CPU destruction
and if all of those are too much for you, go for a palimino core AthlonXP 1900+ - 2100+. (these are getting pretty cheap now days)
But really your CPU doesnt need
an upgrading for a while, dont bother unless your extremely serious about this, and using that computer for some jobs.
as for ram... 512 is satisfactory, but 1 gig is better
if i was you the only thing ide upgrade is my video card, and maybe if you been saving that allowence for a while another stick of ram
good luck man
... Intelligence is irrelevance !!