I`m trying to buy a new graphics card for someone. If I buy an x8 card will it work on a x4 motherboard, obviously at reduced capacity?
Probably, depends on the card. Alot of the last generation (Geforce FX, Radeon 9xxx) where compatible with AGP 2x, 4x and 8x. However, some were only compatible with AGP 8x and 4x, but that should be ok for you. As for the Geforce 6xxx and Radeon Xxxx AGP cards, no idea. Probably a good idea to check out the card manufacturer's website to check before you buy anything though.
*Rambling deleted* for sanity's sake and relevance. Sorry for rambling, I haven't been clear at all with that lot. Bottom line is that an AGP8x card should conform to the AGP3.0 spec which means it will work perfectly in an AGP4x mobo slot (AGP2.0 spec). It will operate at full speed but the speed out of the AGP slot is reduced to the 266mhz of AGP4x which brings virtually zero perf penalty anyway. Both AGP4x and AGP8x cards use the same 1.5v interface, the only voltage difference is with the signalling where AGP8x cards (AGP3.0 spec) use 0.8v instead of 1.5v. Cards are designed to fall back as required, the only thing that usually caused a problem was when the AGP slot can't provide the required amount of power but that's going back to S.Skt7 days (with 500mhz CPUs). The only time you are likely to get incompatibility with remotely modern hw is with old AGP1.0 cards (eg 3dfx Voodoos) which use 3.3v and would either require an old AGP slot (eg AGP2x) or a Universal AGP slot which accepts both 3.3v (AGP1.0) and 1.5v (AGP2.0 & 3.0) cards ... but those are rare. You should find mobo AGP slots are keyed to prevent incompatible cards being inserted and many modern mobos also have a little LED near the AGP slot too. There was a breif incompatibility with the first VIA AGP8x slots and the first AGP8x cards (Radeon 9500/9700) which required the user to disable AGP mode, I'm certain drivers and BIOS files solved that issue long ago.
