Warning: all parts mentioned were purchased used, so there's no need to point out that I can buy better products.
I run a pair of 1440 x 900 monitors that are identical and have DVI and analog input ports. I run them as a single desktop; not mirrored. I was using a Radeon 512MB (a recent model with 3 outs, analog, DVI, and HDMI, and I don't remember the model number) and was dissatisfied that one image was analog and the other DVI.
I went to a mall of used parts dealers and I was told they would help me switch to running 2 DVI images. They sold me a Radeon HD 2400 Pro 256MB and a DVI 59 pin cable that splits into 2 cables. It was the only card that had this type of output to be found so I didn't question whether 256MB was enough.
On Windows 7, so far it looks like I'm not missing anything from having only half the memory. (I will reboot to Ubuntu 12.04, shortly to see if all is well over there.) Given my usage is mostly Microsoft Office and a browser, is there any practical difference between 256MB and 512MB?