oh ok. Well I've had a hunt around, apparently dells are pretty bad for this. It is reserving memory for graphics but it is still available, even though it's not showing in task manager. Basically the BIOS assigns a bit to a segment of the RAM saying that it's graphics memory, therefore task manager doesn't give a crap about it
. However the RAM is still just RAM and as long as you're not using it for graphics, it'll still be available.
Do you have a dedicated graphics (nVidia, ATI)? if so, try changing the graphics setting in BIOS from default to PEG. I don't know exactly what graphic setting, just one that has an option called PEG.
Edit: keep forgetting to include these bits. If you find your display not working after this, reset your BIOS. That is, take the side of the case off, find a small plastic peg placed on two pins with a bare pin next to it. It should have something like CMOS_CLR or BIOS_CLR written on the motherboard next to it. Move the peg one pin over so you cover the middle pin and the pin that was previously open, leave it for 10 seconds then move it back. That'll put everything back to normal.