My HD4870 512 card runs this pretty well. I have some of the stuff on Medium (ground textures I think otherwise my memory usage would be over 100%).

I have same settings and 4870 card with 512 memory aswell. I have also bloom disabled (Mainly because I dont like it). Only slowdown (was still playable, but not enjoyable) has been in one campaing missione where there were several planes (10ish) dogfighting over trenches (Many planes + flak + explosions + lot of smoke was too much)

But what is causing this slowdown:
1]. the lack of Videocard memory so you need a HD4870-1Gb
2]. or the GPU can not handle it, so you need a better GPU, the ATI HD4890-1Gb????

Yes, a faster card with more memory would help but the gain would be minor difference and not worth the cost to upgrade if you already have 4870 512mb. I only upgrade to true next gen video cards and not revamped versions with more memory. That's a suckers game.

I am running RoF on a 1GB HD4870. It is running very well and looks great. I was getting some lockups but it was fixed by adding extra cooling to my PC case.

Without further testing I would say problem was GPU/CPU (I only have E8400 Dual core). Im basing that in a fact that game runs great with only 10 planes around (far from trenches) or that flak + smoke around (no other planes or only 4-6 max). It was the flak + smoke effects when added to those 10ish planes when slow down started to happen (Also I was quite low so game drawed higher LOD to ground units, etc). Also game uses only around 50% from card memory in my settings (high ground textures needs memory most and you cant even use that with only 512 memory).

Oh, and I use vista so that itself is worse platform to gaming than XP