Hello James, welcome to PCHF.
This sounds like you currently have onboard graphics. What you can sometimes do, and looks like in this case, is share your
RAM with video memory. You can specify how much video memory you would like to take from system memory and allocate it to video memory in BIOS.
A new graphics card would mean you can free all of that memory, and allocate it back to your system. The new graphics card you buy will have it's own memory and there isn't any limit set. The only limit is what graphics card your motherboard will support (PCI, AGP or PCI-E). Geforce is a good choice, but please don't think more memory is better. Speed is much more important.