I have a mediocre GPU but loads of RAM, is there any way to gain more performance

Both ;)

You need RAM to hold the data - polygons, textures etc. More ram will allow you to show more detailed textures and run at higher resolutions.

You need a good GPU to do all the calculations needed for a modern game - lighting, physics etc. The better the GPU the more of these calculations you can do so you'll get better effects and it will have an impact on the resolution too.

Check the specs of those cards and then you need to decide what's important - resolution, effects etc. and go for the card that's better at those things.

(I have rectified my answer. To clarify: every process gets 2 GB of virtual address space. All processes combined can't exceed 4 GB of physical memory)

Actually, yes. There are many misconceptions about the memory limit in 32-bit Windows OSes.

First of all, when I say RAM, this includes every kind of RAM. So not just regular, but also your video RAM. Only physical memory though, not swap/paging.

The maximum available amount of RAM per process for a 32-bit OS is 2 GB. If you have a 64-bit OS, a 32-bit process can allocate up to 4 GB.

Additionally, the entire OS can only address 4 GB for every process combined, while 64-bit OSes can allocate 8 TB.

Also interesting is that this limit adds up to the paged pool. This means that a 32-bit OS cannot use more than 4 GB total, which means 6 GB for all applications including virtual memory. Of course, this has nothing to do with RAM, because the other 2 GB would be stored in the paging file, limited to your hard-drive's speed.

For more information, see Memory Limits for Windows Releases.