Mob. x600 is just the 9600 or 9700 (that means both the old 4 pipe cards) as PCIe version. It only gets more ram, it's not really faster.
The first real increase in performance is X700 (8 pipe X800 architecture) or x1600 (4 pipe x1900 architecture with 12 pixel shader processors). Those are already available in different mid-price notebooks.
My OLD System, now Mediastation:
Asus A7N8X Deluxe with Barton 2.5 @ 200*11
2x512MB Corsair CL2 PC400
IBM Deskstar 307030 30GB HD, Ultra ATA 100, 7200 RPM + WD something
60GB HD, Ultra ATA100, 7200 RPM + WD Caviar 120GB SATA133 7200RPM + Samsung 200GB Sata150 7200
Powercolor Radeon 9800pro STILL running 4.3 cats, and Arctic Cooling VGA Silencer
Onboard network cards; !!!Win98SE!!! (yes 200GB HDD on Win98 works fine :))
NEW: HP nx9420, T2400, x1600 256M, 1440x900, 1GB, 100GB HDD, XPPro.