First off, mame dones not use the video card for emulation. However, mame can use the card's hardware for other, non-emulation stuff, depending....
Mame, by default, uses the directX 3D card hardware:
- full screen resizing/scaling/aspect scaling
- overlay, underlay, bezel & "effects" (like CRT simulation overlays)
- UI menu rendering
- multiscreens
- I think some of the rotation (although might be just the overlay stuff)
- I think vsync and "triplebuffering" (FYI, mame does not do true triple buffering, but actually "render ahead" buffering)
By default, mame does not use any directX 2D hardware. However, if you change from "video d3d" to "video ddraw", the above features will stop using the card's 3d hardware and use the card's 2d hardware instead.
If OTOH, the option is set to "video gdi", mame will use the CPU to do the above features (if at all).
So depending on your settings, mame might use 3d card non-emulation accel, or 2d card non-emulation acel, or not use the card hardware at all.
FWIW:
To see if your card is "fast enough" for your current option setup, run a game unthrottled for x number of seconds ("mame game -nothrottle -str 120"), and note the percent speed. Then run the same game with no video ("mame game -nothrottle -str 120 -video none"), and note the percent speed. If the difference is small (0-5%), upgrading your card will make basically no difference with your current option setup.
Note that the above paragraph is for testing your setup, not for benchmarking purposes. And if you haven't bought a card yet, you can't do it or course. And changing you settings and/or mame version can make a difference.