It will give you better 2D support, no doubt about it. Normal GUI stuff tends to rely on drawing ractangles and stuff that the gfx chip can do by itself using it's blitter.
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You can expect higher resolutions and more colourful displays than AGA. Even a basic 2Mb card should be able to deliver 1024x768 in 16-bits (which needs about 1.5Mb of framebuffer).
RTG games should benefit also, but that would ultimately depend on the bus speed and resolution used. For instance, some doom clones ran faster under AGA than they did on Z-II graphics cards since on these systems writing the chunky pixels was limitied by bus speed (even with AGA C2P) so AGA won out there.
Given the ateo bus is a modified ISA bus, I don't know how fast it is capable of.
The pixel64 offers no 3D acceleration of any kind so it won't accelerate 3D games directly, but the chunky pixels should help (unless the bus is slower than AGA, eg Z-II comparable).