lol i have the same exact videocard
hey i just got the asus geforce 6600 for christnas lol. I have the 128mb version though. Just get the 128mb if it is cheaper. If the videocard runs out of memory it just uses the system memory so you will be good if you have a 1 gig. I have a 512mb stick but i am planning on getting an other 512 stick later. For 30 dollars more it isn't worth it for the extra memory. Just go with the 128mb. I have the 128mb one and it works just as good there isn't much difference in performance. If you got questions about the videocard just me. Hope this helps.
P.S. I am pretty sure about the vid card using system memory but someone can double check me on that. I swore i read it somewhere.
New build in progress, but this is what I got for now:
-AMD x2 5200+
-Asus M3A78-EM AM2+
-Buffalo 2x1gb ddr2 800
-Hitachi 250gb Sata HDD
-Antec Minuet Case w/ 350w psu
-Windows 7 Home Premium 64 bit