PCX-2 : You Will Hate This Card. Drivers for Win9x only, alpha blend only, and I think it's the card responsible for most 1998 games having to look like crap. Supports SGL which Unreal can do fine and a bunch of other exclusive games. No OpenGL support, that's just horrifically wrappered for Quake, Quake2 and Hexen2. I had a fan page in the works since its limitations and broken drivers lead to curiosities.
Game potential: Up to early 1999, I think Midtown Madness was the last game to have special support for this chipset.
Neon250 : Never tried it, it has PCX-2-ish drivers of similar compatibility. Supposed to have all the blend modes this time around though. Never had it, and was a flop so it was hard to obtain in the first place. Not a lot to write home about, don't buy into the lame delusional hype Dreamcast fans tell you about this 'great' PowerVR chipset (same chipset used in DC, but in card form and arrived way too late).
Game potential: Up to mid-2000, I THINK DirectX8 killed this one. I never seen any neon250 compatibility notes since then.
Kyro 2 : Probably the first 'normal' card that's up to scratch in basic TNT2-ish compatibility with OpenGL and Direct3D, a very VERY good card for retro gaming (16-bit dithering is a sight to see, very little graphics corruption, SDL friendly, and IIRC supports paletted textures). It HAS NO SGL SUPPORT AT ALL, so don't expect to play any of the old PowerVR exclusives on it, and the old minigl wrappers fail on it as well. It is also the only card with XP drivers and Linux drivers. One of the features it is hyped for is its 'hidden surface removal', which did jack squat to help the poor framerates and everything uses visplanes/lists/trees to avoid overdraw via software anyway.
Game potential: Up to late 2002-2003. By then hardware T&L was taking off, the Kyro2 couldn't do that and the popular thing to do was to SLOWLY EMULATE T&L via 3d Analyze! but this lead to many anomalies. Also it has hard freezes with UnrealEngine2 games. Well, at least the fillrate and texture units were good. Bloom and fullscreen framebuffer effects will slaughter framerate, so not very next-gen proof even if it ever had HW T&L or even pixel shading.
The drivers of all 3 cards aren't very optimized, they WILL DEMAND A GOOD FPU so don't do something stupid like put it in a Cyrix or a 486. A Pentium II 233 is the target system for this.
You can also use the PCX2 and the Kyro2 at the same time without a driver or hardware conflict. They won't WORK TOGETHER, but they can work, so you can do your feasting on the same machine. Windows 98SE *highly* recommended.
As for PowerVR... you know the iPhone? Yep, that's PowerVR all over the place. Mobile market is what they're playing dominatrix as now.