How much video memory do I need for Office and Youtube usage.

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?

Sounds like a system providing RAM for a video card that hasn't enough of it's own would be slower than otherwise. Fortunately I also added an extra 2GB of main memory today.

Good idea, well executed! What does this make your total RAM?

The total RAM is now 4 GB and it cannot be any higher on this motherboard which is a Dell G45. I saw my Windows Experience for Aero Graphics go down from 4.3 to 3.9 upon installing this new card (Radeon HD 2400). I didn't notice any difference though. I am beginning to think the Window Experience ranking for graphics might not be relevant for non-gaming. However, it is relevant to the computer industry's profits because it may convince people to upgrade even when not compelled to through a practical need. Feeling a little impractical I swapped in a Radeon HD 5450 and now Aero Graphics is rated 5.0. I had to get yet another adapter (Display Port to DVI).

Ah, 4GB should be plenty for shared graphics, as I used to only have 3GB and all my graphics were shared.
You'll be fine, especially as you mostly use your computer for Office and Youtube.

-Spaz