Phil
The SiS 661FX is, as far as I’m aware, on onboard video card (ie built onto the motherboard of your PC), and as such is likely to be perfectly adequate for 2D apps (Office etc), but next to useless when it comes to displaying 3D graphics.
Open GL & DirectX are just 2 different APIs (Application Programming Interfaces) that are used to display 3D graphics. All the message you got means is that your graphics card drivers don’t support the OpenGL standard. If it works okay in DirectX, then the image quality is likely to be very similar.
Your question asking ‘is the video display likely to be better?’ is a bit vague. If you mean, will the picture look prettier, then no, not really. If you mean will another card be more powerful (ie smoother, able to run at higher resolutions etc), the answer is almost certainly yes. Even a cheap modern graphics card will easily outperform the 661FX. The main problem you may have is that as you have onboard graphics, your motherboard may not have an AGP or PCIe slot, in which case you’ll need to hunt down a PCI card.
It’s difficult to recomment a card without knowing your requirements & your budget. There’s no point spending loads on a monster graphics card if you never play games…