You said you wanted to game at Max Setting @ 1080p
1gb in SLI isnt gonna help much just smoothen out the game play with your current settings
If the 7870 is too price. I say buy a 7850 now and get a second one if you need to down the road. 7850 can be OC to 7950 levels with luck but you should be able to match 7870 levels with a OC. 7850 CF can be run in a 550w PSU and performance at 7970 GHZ levels.
Just something to consider if your strapped on cash. Buying another 460 will just be dumping money since you want Max Setting @ 1080p