welcome langsons,
well i don't want to sound sloppy, but 1 GB ram sticks (and i'd assume DDR2 in your case) don't cost a fortune ....
the facts: 3 GB RAM minus 1 GB for windows = 2 GB for user space (minus something for other applications also needing user space)
you can't leave less than 1 GB to kernel space ... the /3GB switch would actually be neccessary, because otherwise windows would divide the available memory into half (1,5 + 1,5) ... don't forget to set /USERVA=2048 (maybe a little bit less) ... and do not just change the boot.ini line but add an additional one ...
christian
edit: in any case i'd recommend to get memory modules with the right layout (double vs. single sided) and the most similar timing to your 2 x 512) - not all chipsets accept all memory sticks in every combination ... check your motherboard manual
and remember: only a CRAY can run an endless loop in just three seconds.