As far as I know, StarCraft II quality is not increased by using DirectX 10 as opposed to DirectX 9. I think XP users can crank up all settings to ultra as well.
I've found this teamliquid thread and this overclock.net thread that both mention the game is only a DX9 game.
Maybe the reason for that rumor is a Blizzard FAQ from 2007 (emphasis mine):
Will StarCraft II take advantage of DirectX 10? What other graphical goodies are included?
The game will be compatible with DirectX 10, and we're still considering whether there will be exclusive DirectX 10 graphic effects, but the graphics engine will also be very scalable to ensure that a wide range of different systems will be capable of running StarCraft II. The new engine is also capable of rendering very large units, as well as large numbers of units on screen together. Havok physics have been integrated into the engine for added realism as well.
The official recommended system requirements, by the way, implicitly say DX10 is needed:
PC Recommended Specifications:
- Windows Vista®/Windows® 7 <-- these OSes come with dx 10
- Dual Core 2.4Ghz Processor
- 2 GB RAM
- 512 MB NVIDIA® GeForce® 8800 GTX or ATI Radeon® HD 3870 or better <-- these cards support dx 10
