heyo
i had the same question for a really long time.
what it really comes down to is: what are your computing needs?
with that info seach these forums and google any combo of these keywords:
mbp 256mb vs 512mb, 9400gt vs 9600gt
using a quick search on these forums i found this
https://forums.macrumors.com/threads/580718/
tons of more info out there.
and here is the link of the MacRumors search engine to use in conjunction with the forum search ><
http://mroogle.*************/
i got the umbp 2.4 256mb model cause all i do is play wow and want to run it on an external display. and the jump in price for me just wasnt worth it.
and the 256mb card is perfectly capable.
and the difference between the two off the top of my head is: .26 in processor speed, 256mb video memory, 2 gig ram, 70ish gig hdd, and 3mb of l2 cache iirc and maybe the speed of the hdd but im not sure about that one.
good searching to you!
☮