Cheap GPU for 1680x1050+1440x900 monitors + compiz

Hi I have a 9800GTX+ and a dual core, I am curious to what kind of gpu performance differences i could expect if I replace my 9800GTX+ with a Geforce 210/610 - up to a 750 (though am looking for a quiet (and cheap) gfx card that I could use for a git server/silent extra computer when I end up upgrading).
My main worry is getting a worse performance because of my high resolution X screen.

I would go with the gtx 750 out of that line up it would be newer and use less power and should handle the resolutions fine its rated for at least 1080p

The R7 260 seems to offer better performance specs wise for that price.
Are the Memory Bandwidth, Texel Rate & Pixel Rate good representations of the card's actual performance? -and therefore the card's theoretical resolution limit?

or 265/370

I had little problems with a tiny R5 230 running three displays. (1680*1050 and two 1280*1024) under Linux and it drove dual HD screens without problems. Windows no problems whatsoever. Following the flawless logic of higher number = better the R265 or R260 should do fine.

If you have the money to spend on a 750 I would look around for a used workstation (HP Z*** series or Dell T**** (the asterisks stand for numbers, for example Z210 or T3500). Prices start at 100$ so there should be something in it for you. There are a lot of used workstations out there that support two display port displays (meaning two full HD displays are no problem).
(I know, I know: That is not what you asked, but I wanted to mention the possibility)

What kind of problems?

I plan to buy a used workstation laptop in a few months and really don't need another dual core desktop.