I'm about to buy a gpu and I don't know how much vram I should get. I want a 7850, and I would need to pay about 20 more pounds If I went for a the 2gb. I'll be playing on a single monitor, no more than 1920x1080.
I see. Well IMHO, game that will come out this year and next year will use more VRAM. So why not invest if you're going to buy now. If I were you, I wouldn't risk going cheaper on my GPU for 20pounds if the games are getting "complicated".
1GB is enough for almost all games that are out, 2GB is for the future or better cards. What is your budget and your PSU, CPU? With 7850HD 2GB you would get around 1-2% more fps and at this resolution 1GB is fine for now.
I view the need for graphics vram much the way I view system ram.
You need enough to keep most of what you need in the vram or suffer the performance penalty of refreshing it.
In the case of system ram, a page fault requires a hard drive I/O access. That takes a relatively long time.
In the case of vram, needing something no in vram requires a transfer from system ram via the pcie interface.
The pcie interface, even the older 2.0 version is considerably faster than the sata interface for hard drives.
Thus, not having sufficient vram is a performance issue, but a very small one.
No doubt, more vram is better. But... how much will performance be improved, and at what price?
My take is to use the price difference towards a stronger graphics card in the first place.