It's just that Mame didn't use the GPU at all, but now it does a little bit. It still doesn't use the GPU for emulation, true, but for rotation, resolution changing, artwork/bezel, etc (if you have video set to d3d or ddraw). Other minor directX 9.0 stuff, too, but again not for actual emulation, and only if you have the setting to use it.
You don't need a super powerful top of the line GPU, any dX 9.0 compatible will do pretty will. Bottom of the line on-board GPUs are boarderline, though, especially when running at higher res and or with artwork on with a Core 2tm CPU. And pre dX 9 can also be boarderline, if matched with modern CPUs. If the CPU is as old as the GPU, though, it's very likely the CPU is still the bottleneck (depending on exact CPU, game, mame version, and GPU in question, of course).
In summery, after 0.106, it's wrong to say "the GPU doesn't matter", but it's correct to say "all you need is a decent dx 9 GPU", or "the GPU won't speed it up, but it can slow it down", or "basically a modern or last gen GPU will be fine", or even "the GPU probably won't matter if you buy a new one".