Yep that does seem abit odd that your not seeing ANY increase in performance then
you still didn't mention OS (is it WinXP? cause it is memory and cpu hungry)
Things that come to mind to try or invetigate
1. Is the memory you have both the same rating? (if one is slower than the other you might be better off without the slower one)
2. Check in your bios setup for AGP settings and see if they are correctly configured.
3. How many other programs are running when you use LFS? Try closing down as many as you can through the task manager.
4. Check you computer for trojens, viruses, spyware, adware, etc...
5. Play with some settings in LFS try:
under Graphics -
Disable rubber - all
Disable flags - all
Disable trees - all
Simple wheels - mirror or all
Shadow type - low res or off
Enable dither - no
Half texture - no
Mip filter - no
HVS - on
Z-Buff - 16 or 24
Simple track - off
Haze effect - Yes or No
under Misc -
Multiplayer Speedup - yes
Multiplayer car Draw - 150m or less
6. If you change all the settings for your video card to get best performance is there any increase in fps over settings from max quality?
Thats about all I can think of at the moment... and its 1am so bed time
Hope you get some improvement, I wouldn't have thought the poor performance was just due to the Celeron