First of all, my specs:

Pentium 4 3.0GHz
1GB of DDR
120GB HD, 500GB external
ATI 2600 HD 512MB DDR2 (AGP)
Mobo - P5P800S

Recently I purchased ATI 2600 HD 512MB AGP and after installing it I noticed my computer started processing much slower than with nVidia 6800 XT 256MB. I went into BIOS settings and tried to change video settings from 256MB to 512MB but 256 was the last and highest there. Does this mean my motherboard doesn't support 512MB video card? I just increased monitor refresh rate from 60 to 75 which increased the speed and looks like it works to its potential, but is there anything else I should do to maximize performace and make sure I get full speed out of this card?

Thanks in advance,

- DimaWhat exact setting did you change in the BIOS?
Make sure you completely uninstall your old video drivers and install the latest ATI drivers for your new card.I didn't change completely I only tried. I am at work right now and I can't remember the name, it was something like VGA, after pressing Enter it allows you to choose your video card memory starting with really low (around 56 may be lower) and going up to 256. Since there was no 512 option I didn't end up changing anything and didn't save the settings. Yep, I completely uninstalled my previous drivers from nVidia.I think what you tried to change is the onboard memory allocation, for the onboard graphics chip.
Let us know exactly what it's called when you get HOME if that's OK.
Is the system overall slower, or just in 3D applications?Mostly windows related tasks, 3D applications perform pretty good. Is the ATI 2600 HD really better than nVidia 6800 XT?

Sure, I'll post the setting when I get home. Thanks guys.At first you said it replaced a 6800XT, is that right or was it a 6600XT?
What kind of memory did it have, GDDR3 or DDR2?Sorry, my bad (already modified). Correct it is 6800 XT, I believe it was just DDR.The 2600 PRO should be faster than the 6800XT.
If it's Windows related tasks that are slower rather than 3D, i think it may be a lack of maintenance.
What maintenance do you do on your PC?I use CCleaner to use temporary and unused files, Registry Mechanic, once every couple of times I'll perform defragmentation. Always updated although recently I had Ad-ware from some MelWare website but it was cleaned by Kaspersky Anti-Virus 7.0 and I happen to FIND large amounts of small files created in different folders probably left from ad-ware but they don't do anything.And so I CHECKED the setting that I tried to change it is Graphics Aperture Size currently set to 256 and thats the highest option. What is this? Does this have anything to do with the video card?I'd remove the CMOS battery for 10 minutes with the machine unplugged and then replace it ...re-boot and start over...who knows how many changes you made in the BIOS that could be the cause of these symptoms.