I was thinking of getting the MX440 @ OM, but notice that it is the PCI version.
What would be the difference between this and the AGP version of the same card?

Really depends on the system the card is going into. In real world tests there is little performance difference between the same card designed as PCI and its AGP counterpart. But, again, it all depends on the card and also the system.

Not sure what the differences would be in this specific card, but the problem with PCI is that it's bandwidth-constrained (i.e., amount of data going to the video card per second is limited). I believe the standard is a 32 bit/33 MHz PCI bus which has bandwidth limitations of 133 MB/s, and this is for ALL peripherals connected to the bus (Sound card, NIC (I believe) USB devices, HDDs, etc.). To compare, an AGP 4x bus has bandwidth limitations of 1GB. AND it doesn't have to compete with other devices for the bandwidth (mainly NIC, sound card and USB input devices).

It doesn't really matter if you're not using the video card for gaming or heavy graphics applications, but I assume you are, since you want to buy a video card. It will make a significant difference especially in multi-player 3D games. Even in single-player heavy graphics games, it will make a difference, as the bus will probably become the bottleneck, especially if your processor is already pretty fast (>1.5 GHz).

F

Something else to think about....
you have 1 AGP slot. And a limited number of PCI slots. If bandwidth isn't an issue and you have lots of available PCI slots, then it may not matter to you. If you have items in your PCI slots and no extra room, then you may want to use the AGP slot.
Add a NIC, sound card, modem or scsi card, maybe firewire, maybe USB card, etc....there are things to add...they all take PCI slots.