Hmmm 4850 or 4870

So, i am currently looking to upgrade my single 4850 videocard. I've already made a choice that i definatly will, but i just checked some prices on newegg and figured out that a 4870 will just cost $20 or so more, which is not a really big deal for me. Now this is how i am thinking this through: 4850+4870 does not have a lead on a 4850cf, however a 4870 does have a good lead on a 4850. Now, i don't really know how many games don't support or just like crossfire, but in any of those cases a 4870 would beat just one of the 4850's running. Also, i am thinking of getting a 4870 1gig, because of some games that require you to have over 512 to put it on all out max (GTA4), so i could always just run the 4870 by itself and get the whole thing... so with the 4870+4850 config i am looking at on demand FPS when i need it, and on demand max quality with the loss of some fps. So,what do you guys think, am i missing something? Any input is appreciated, thankyou!

Bouncing in and out of CF is an interesting strategy. But what resolution do you play at?

Then a single 4890 would be good