The xbox actually has 256 MB of GPU ram.
The minimum requirement for PC is 512 MB. The recommend is 1 GB.
This is probably because they know Pc gamer's don'tg ame at below 720p. And the recommended will get you playing at high settings. According to Bethesda the PC version cannot go below console settings, therefore low on PC = console.
I assume then that "high" probably means higher resolution textures, larger view distances, mroe objects ont he screen, etc. Which naturally requires more GPU buffer.
The gaem seems decently optimized. The minimum requirements are almost the PC equivalent of an xbox 360, and the recommended look pretty decent as well.
I wouldn't worry.
What are your specs?