I know you specified 256M video, but the Lenovo X Tablet is really a nice little PC that is rugged and pretty impressively designed for ease of use and it seems to share video memory pretty good. For everyone that I know who has used a few tablet brands, IBM/Lenovo has the best and one of the most expensive. Due to that reason alone, right now I am looking at the HP tablets which are considered very good and affordable. I have not heard of anyone having a good experience with a non-namebrand tablet and the ones I have found are not economical. However, I really suggest you find various tablets and try for yourself because, unlike other PCs, each tablet is pretty unique for feel and features.
UPDATE: Never mind, I just read that HP/Compaq are out of the tablet business now. So I probably will not be buying the one I am looking at.