You will see a lot of GeForce 7300 LE-based cards with 128MB of on-board memory, supporting 256MB of memory in a 512MB system. You will also see GeForce 7300 LE-based cards with 256MB of on-board memory that support 512MB of memory in a 1GB system."
That sounds like the video card is borrowing memory (using shared memory) according to what your total system memory is. If you have only 512 MB of system memory, the video card will use part of that as video memory for a total of 256MB. And that there are other 7300LE cards that will borrow enough system memory to total 512 MB if you have 1GB of system memory in your computer.
However shared memory is much slower than memory that's built into the video card, and some games will not work with shared memory at all -- despite what certain video card manufacturers would have you believe.
If you need to buy a new video card, you may also need to buy a power supply that can support the new video card. We can't really advise you without knowing more about your computer -- like the wattage of your power supply and what type of video card slot you have (AGP or PCI-E). If your computer has a model number, we might be able to find what power supply shipped with it.