Okay, do you know the specs of his power supply? If his power supply is weak and/or a cheap power supply his choice in graphics cards will be limited. How many watts the power supply has and amps on the 12 volt rail is important to know
As for your question about the different slots PCI, AGP, and PCIe.
AGP or Accelerated Graphics Port is basically the old slot for graphics cards, introduced in the late 90's if I'm not mistaken. There are 4 versions, AGP x1, x2, x4, x8. The x8 offering the max bandwidth available in an AGP slot (2000MB/s). It provides more bandwidth than a standard PCI slot, allowing for better performance in games. However, it doesn't have the bandwidth of PCI-e. No new motherboards are being made with AGP slots because it is outdated technology. The best card available for AGP is the ATi 3850.
PCI is the most common peripheral connection port. However, due to the lack of bandwidth (133 MB/s) it is not a good choice for graphics cards. PCI ports are commonly used for add on cards or peripherals such as sound cards, network adapters like ethernet expansion or wifi cards, usb/firewire add on cards, and so on. I don't know the best available card for PCI because for the life of me I don't know anyone who would use PCI for a graphics card
PCI-e is the successor to PCI as of 2004. It has many available port types PCI-e x1, x4, x8, x16, x16 2.0. PCI-e x16 offers the maximum available bandwidth for any peripheral port. (PCI-e 2.0 to be exact) What's great about PCI-e is the compatibility of the different port types. A PCIe card will physically fit (and work correctly) in any slot that is at least as large as it is (e.g. an x1 sized card will work in any sized slot) PCI-e x16 1.0 and 2.0 are what current graphics card manufacturers are making their cards to fit.
I hope that sort of answers your questions without going into too much detail.
