While the memory might hold you back some you wont get close to 12000 points by upgrading just the memory, nope. Its the gfx card that is holding you back.
On my system i get around 9500 points with default 3dmarks settings and I use fast ram.
these are my specs: Xp 2400+ 512mb PC2700 Gf3 ti 200 @ 230/530 and a GA kt400 mobo.
A friend of mine with a xp2000, 512mb 266mhz and a ti4600 scores about just above 11000. So Im 100% sure that it is the gfx card...