Ram Disk...

I don't know that I understand what you're trying to achieve. A RAM disk is taking real, physical RAM and having the computer pretend as though it is long-term storage (that happens to be super-fast). An SD card isn't super-fast, and already is long-term storage (no pretending required). It sounds as though you're asking how to turn an SD card into... itself. If you want to do the opposite (take the SD card and use it as though it was additional RAM)... isn't that what ReadyBoost is?

The situation is worse for video memory. Video RAM is faster than regular RAM. I'm not aware of any way to treat an SD card as though it's video RAM (not that I've looked), but it sounds like an awful idea. It wouls be orders of magnitude slower than the video RAM and would at best cause severe slowness, and at worse cause bugs/errors/instability.

I don't think you can do what you want. ReadyBoost will support a 4GB extension of "disk cache" on a USB or SD card, but it doesn't extend your available physical memory...

Unfortunately with 32 bit OS, you are limited to 4GB of addressable memory. purchase/upgrade to 64 bit and max out your motherboard if you so choose (Win 7 64 bit supports up to 192GB for Ultimate, Enterprise, and Pro. 16GB for home premium, and home basic 8GB). The maximum will also depend on the motherboard.