Hello guys,
I hope you all are doing good. I need your help as quickly as possible please. I currently use Fedora Core 3 and TurboLinux 8.0 Workstation. The major problem with these two distros on my computer is that none of them seem to provide resolution other than 640x480. I tried changing resolution to 800x600 in X-Configurator in FC3 and turboxcfg command in TurboLinux. after restarting the computer and booting up linux , at the configuration panel it says the resolution is 800x600 but in reality it is no different than 640x480. If I try using 1024x768 it works but only with 250 colours. If I select 16 bit and restart again it shows in configurator that resolution is 1024x768 in 16-bit but actually its 640x480. This has happened with all the 10 linux distros I have tried but I dont want to give up on Linux. The following are my computer's configuration.

Intel Celeron-D processor 2.4 Ghz
256 MB RAM
Mother board is Mercury PI845GLM-AGP which has onboard grapic accelerator with 64 MB memory
and I have a 15-inch Samsung SyncMaster 591s monitor.
Please help.
P.S.: In windows I am able to run desktop at 1024x768 32-bit resolution at the max.