HD4870 Sluggy Performance ????????

Hi,

AMD Phenom II X4 955 BE
ASUS M4A78-E (790GX)
ASUS RADEON HD4870 1G
4GB Kingston DDR2 800Mhz
320GB SATA
Windows 7 Ultimate

Playing Undercover on below setting
Res: 1024 X 768
Detail: Max (High)
V-Sync: OFF
Anti-Aliasing: 6X

The FRAPS count
Min FPS: 24
Average FPS: Between 26-32
Max FPS: 60 (Few times it hit such FPS)

Making Anti-Aliasing 2X improve + 5 FPS.

isn't it bad performance for such system ???????
Is it because I play DX9 game on windows 7 (DX11)?
Playing DX9 games on windows 7 means playing it on DX11?
Help confused by this performance.

Regard