Well like enewmen said the difference is large but u wont notice much of difference in screens below 22"...
And the 260 comes wit 896MB default...
And about the 4870 consideration, it purely depends on wat u like...but the mobo plays a role in this...if u have a crossfire support mob, then its logical to go wit the ATI...but if u don plan to crossfire then the GTX 280 is a better option. But nt worth if u have a monitor size less than 22"