Is it possible to disable half the RAM on an Nvidia video card

I understand what you're talking about (rumtscho and TomWij don't understand your question correctly), although I'm not sure how much of a performance increase you're going to see from having a few hundred more megabytes of RAM. Are you really using up all of that RAM all the time?

That said, if you really want that extra RAM, you really only have a few options:

1. Use a soldering or desoldering iron to remove half of the RAM chips. You risk making the card completely nonfunctional if you do this incorrectly. Look and see if the chips are marked or numbered on the board. If so, remove half of the highest numbered chips (for example, if you have 8 RAM chips, remove 5-8 and leave 1-4). As long as you don't mess up the board when soldering, there's a fairly good chance that this would work. (Of course, most VRAM nowadays is surface-mounted TSOP or similar, which could be very difficult to desolder).
2. Try to modify the BIOS or the card settings to disable some of the RAM. This is probably possible, but would require some hacking. You'd want to start by dumping the BIOS with a BIOS flashing program, then try to reverse engineer/disassemble the code and make modifications, then reflash the card with the modified BIOS. This is something the manufacturer almost certainly wouldn't approve of.
3. Buy a card with less RAM, or try to get a friend to trade their video card with you.
4. Get used to having less RAM. Maybe disable some startup applications, or try running a few less programs at the same time? It really isn't that hard to get along with 2.8GB rather than 3.2.

Basically, there is nothing you can or need to do about it.

I don't quite understand if you have a real, separate video card or a "shared memory" video card (the latter is common in some laptops). Your first sentence sounds like it's separate, but the verb "reclaim" makes me think that you mean the second option.

In the first case, the RAM of the video card is not the same as your system RAM. They are not only different chips (the system one gets plugged into the mainboard, the video RAM is soldered to the video card), they are also different types of RAM. So there is no way to let the system use the video card RAM as additional RAM.

In the "shared memory" case, the video card doesn't have its own memory, and uses a part of the system memory for graphical purposes. But these cards are made smart enough. If this is what you have, your VC isn't always hogging a full gig of your system RAM, it only uses as much as it needs. So there is no need to do anything.