Is there any point in adding more RAM or...?

No prob. Thanks for trying. :)

While adding RAM is usually a good way to see an overall increase in performance (Windows uses less Disk cache which is very slow), video playing is more affected by the capabilities of the CPU/GPU combo.

The GPU (even when embedded in the processor) can help a lot and take care of most of the decoding process. But it needs the correct features for that. Looking at the specs of the G33 Express (ARK | Intel® G33 Express Chipset (Intel® 82G33 Graphics and Memory Controller)) it has Intel Clear Video. This technology will help offload video playing to the GPU but only up to a point. It needs the video to be the right resolution (afaik the G33 can't help much with 1080p videos), the right encode, and also the right media player (you could maybe try Home ? MPC-HC which is lightweight and supports hardware acceleration).

You could also check Linux on your sister's PC. Most Linux distros are better on old hardware than Windows (less memory used, less OS footprint). Ex: Main Page - Linux Mint. It can be run from a bootable USB without installing, just to get a feel of the OS.

I ordered a new ( not used ) GeForce 6200 card for $19.35 on Ebay today. :) Thanks guys!