Texture painting an image file.

Hi everyone.
I am trying to texture paint on a 4096 x 4096 image file to use for an entire medium sized scene for a mod i am making.
I know this will work for my project because i did a small test with hardly any detail and there were no issues in game when i imported the terrain model along with the image file.
The problem is that texture painting an image file this large is very slow in real time viewing.
I believe this is because although i upgraded my system recently i am still useg an on board graphics card with only 256mb ram assignied to it.
If i am right an in need of a good graphics card then i need some advice on what to buy.
I have the money, and need to know what the specs should be for this graphic intensive work.

Any advice appreciated.