I have the same problem, SimCity 4 runs like crap on ATi cards (X1K, HD series). This is what I found:
Test Systems:
AMD Phenom X4 9850 BE
4x1GB DDR2-800 C4
MSI K9A2 Platinum
320GB SATA2 WD
OCZ Evostream 720W
Windows XP 32b SP3
Intel Core2 Quad Q6600
2x2GB DDR2-800 C5
MSI P35 Neo2...