Despite what the naysayers here will tell you, there is a way for you to mine with some eficiency on that hardware.
Now, motherboards of that generation will have only AGP and PCI slots, no PCI-E. The best video card that is suitable for mining and available in AGP form is a 4650; it can be found on a few websites, cheapest I found is here
http://www.geeks.com/details.asp?invtid=HD-465X-ZPF2-R&cat=VCD(actually google shopping has a few hits for 4670 AGP, but they're all over $120 - not worth it IMHO for an extra few Mhash/s)
According to
https://en.bitcoin.it/wiki/Mining_hardware_comparison you can expect around 30Mhash/s from it, which will make you about 0.015 BTC/day. It's about the same hash rate I get from a geforce 8800GT.
(edit) Newegg has an AGP 4670 for $99.99 after rebate if you're so inclined:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161337(edit2) There are also a few PCI (not PCI-E) 5450 cards at newegg for around $100 a pop, each predicted to give a whopping 15Mhash/s or so.
