PCI-express is the new format - it has a physically different connector than AGP. Your motherboard will prob be an AGP motherboard as you have a Geforce 4 MX. Therefore you physically cannot use a PCI-express card.
PCI-express can transfer data to and from the card faster than AGP can - however, only if the videocard and computer can keep up, which currently they can't! >;o) Therfore, in the future, PCI-express will be better as the cards get faster, and AGP will become obsolete. However, currently, you are dictated what type of card you buy by the physical slot on your motherboard.
Be very careful you don't get PCI-express confused with PCI. Original video cards used the PCI interface - probably those cheap 256MB cards you've seen. These are slow. PCI is again a completely different slot to PCI-express - they just have a similar name! PCI has basically been obsolete for the use of videocards for a while now, although slow GPU's of each generation are released for PCI for people with ancient computers (like me!), although PCI is still used for many other cards like modems, sound cards etc. These will soon be replaced with PCI-express X1 slots (the video card PCI-express is usually a x16 slot, as opposed to the AGP x8).
See ya.