I would suggest that instead of trying to upgrade the current motherboard, you get a motherboard bundle and replace the whole lot, you can still use your existing Monitor,harddrive,mouse, keyboard etc, however your PSU (Power unit) will proberley need to be upgraded as your new board wil have Sata connections as well as PCI.
The advantage is they are ready built- all you have to do remove the old board and replace with the new.
I am guessing that your current motherboard has AGP graphics and PCI slots for the drives (Wide grey cable).
There are very few motherboards that have an AGP slot for graphics now (most are PCI-e)
Have a look at Microdirect, Novatech, or just search for 'Motherboard Bundles' to see what is available at at what prices.
Advantage- You will have an up to date computer that should run most programs.