Which GPU do you have now? We could give you more details if we knew that.
First off, the starting point for NVidia drivers is the NVidia Driver Download page. Just websearch “nvidia download” and you’ll come right to it:
Now, select your GPU model name along with “Windows 7 64-bit”, and you’ll get a download link to the latest driver version which supports your GPU on that OS.
By all appearances, they support some pretty old NVidia GPUs on Windows 7 64-bit (e.g. GeForce 9), so you can likely find a GL driver for your existing card.
Any driver you download should support OpenGL 2.1 (since that version of GL is so old). For OpenGL 4.0 support though, you’ll want to look for an NVidia GeForce 4xx or newer GPU (excluding the GeForce 405).
If you do need to purchase a new GPU, check out 1) what socket type/version it needs to plug into (e.g. PCIe x16), 2) what power connector(s) (if any) it requires, and 3) its max power draw. Then verify that your motherboard and power supply can deliver that with power margin to spare. If in doubt, start with a cheap GPU with low power draw and no external power connectors required. In any case, you can double-check the GPU’s capabilities and what max OpenGL version it can provide here:
and on NVidia’s web site.