Do I need a separate OpenGL installation/upgrade or has it to be included into my graphic card driver? Where can I get it?
OpenGL can't be "upgraded", and it must be included with your graphics driver. OpenGL exposes GPU hardware capabilities to the operating system. If your GPU hardware were more capable, it could support later revisions of OpenGL on Windows XP with a newer AMD Catalyst device driver package. But your hardware is at least 6 or 7 years too old to support that.
Now, emulation of any revision of OpenGL on the CPU is possible. However, the reason that GPUs exist is that they can compute graphics operations much faster than a CPU of the same generation. For any game, whether it's simple 2D or not, my rule of thumb is, you can run its 3d renderer in software with a high-end CPU from 10 years in the future. So for an OpenGL 2.1 game, it would've had to have been released no earlier than July 2006 (since that's when OpenGL 2.1 itself was finalized), meaning we could start to see playable framerates (30 - 60 fps) when rendered in software with an optimized OpenGL 2.1 implementation on a current-gen CPU.
Of course, on current-gen Intel and AMD CPUs, we almost always have an on-die GPU that's more than capable of running an OpenGL 2.1 game, anyway, so...
Indeed, the mesa3d project maintains such an optimized software renderer as part of its project, but support for Windows is an afterthought, and building it is a bear. Additionally, they don't guarantee good performance or even correct behavior unless your CPU has SSE2 and SSSE3 and SSE4.1. A CPU contemporaneous with a motherboard with an AGP slot (something like a Pentium 4, yeah?) may have SSE2, but probably not the others.
Unless you have a high-end, modern CPU from the past 3-4 years, like a Core i7 3770K or better, you're not going to be able to emulate OpenGL 2.1 in software at a playable framerate. Not even for a very old game. Given that your CPU is probably as ancient as your GPU, you're completely out of luck. That GPU is completely "fixed-function" and does not support user-submitted code. OpenGL 2.1 runs almost any arbitrary operations on the GPU when submitted as a fragment shader. You can buy an ARM System on Chip for $1 with a GPU 1000x better than that RV280.
You're better off upgrading.
OpenGL support is entirely left up to hardware and driver support. You first have to determine if your hardware supports it. – Ramhound – 2015-09-08T12:07:16.527
It may be time for both hardware and OS upgrades ;P – SnakeDoc – 2015-09-08T14:42:37.290
@SnakeDoc Not really, as that's not my main machine but the one I keep to save all my old games from death by incompatibility with newer systems. I don't use it any more except to enjoy some old classics! :D It's totally offline too, so no risk from that side either. – Byte Commander – 2015-09-08T14:46:30.613
Well, in that case, although I hate to suggest it, maybe an XP VM running on a newer OS with newer hardware would do the trick. VM's usually aren't the thing you want to use for gaming, but if you must keep XP and want to play newerish games, maybe the best option. -- As an aside, have you tried running the game on a newer system but in XP compatibility mode? (right-click the game.exe --> Properties --> Compatibility: XP --> now run the program). – SnakeDoc – 2015-09-08T15:48:35.727
@SnakeDoc To be honest, XP was the last Windows OS I really liked. I prefer Ubuntu nowadays. – Byte Commander – 2015-09-08T15:56:33.253
I'm a Fedora guy myself ;-P – SnakeDoc – 2015-09-08T16:29:47.593