Geforce4 MX420

If your computer only has a PCI slot, don't bother upgrading it because even if you do, it won't run anything recent.

Just sell it on craiglist for like $100-$150 and get new components for a new computer off of newegg or microcenter.
 
Depends on your system and what you're going to use it for. If you just want a cheap video card then a GeForce 5200 would work just fine

EVGA GeForce FX 5200 128MB 64-bit DDR AGP 4X/8X $29.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130210

or if you wanted something faster to play some old games this is just a few bucks more

SPARKLE SF8855DT GeForce FX 5500 256MB 128-bit DDR AGP 4X/8X $34.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16814187043

Well even the 5200 would be generally faster and better than that MX420. If you're looking to buy something locaklly any cheap Radeon 9250 or GeForce 5200 would do. Some places may even have an MX4000 laying around, which is pretty much the same thing with faster clocks.