I don't think you should give up on high-rez texture packs.
Just because they won't work well on an APU (as I demonstrated), they work fine on almost any decent GPU with some dedicated VRAM bandwidth. Even entry level cards with GDDR5 have about 4-6X the memory bandwidth of an APU, and will play 256X textures just fine.
The same texture pack that brought the APU down to the 10-20FPS region, runs on my ole GTX460 768MB edition at 60-120FPS (sig rig).
A $60 G3220 combined with a $100 ($80 after MIR) HD7770 ( http://www.newegg.com/Product/Product.aspx?Item=N82E16814161402&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
) is very close to the cost of an APU, yet will yield mine-craft frame-rates 4-6X higher, which is plenty to run a lot of the HD texture packs. I don't want to promise 512X texture packs but it's possible they might be playable as well. (I haven't tested them on my GTX460-768MB, but if you'd like me to, it should be a similar performing card in minecraft to the HD7770).
The APU doesn't make a lot of sense for this build to me at all, when there are many discrete CPU/GPU combinations that can compete price wise with the higher end APUs, while offering substantially better performance (like 400% improvements).