Linux newbee - No resolution available other than 640x480:-(

[rajdumbledore.bizhat.com]

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.

[veera.bizhat.com]

Since you have onboard graphics chipset, try changing video memory alotted to video in BIOS to 64MB. \var\log\XFree86.log would show more details about what is happening actually. If possible paste that log here. I would try to solve your problem.