PCI Express (PCIe) is vastly different than AGP or PCI and PCI-X (for eXtended). PCIe is based on a number of lanes coming from the motherboard chipset, with each lane sporting 250MB/s in each direction, or 500MB/s total.
PCI it's bigger brother PCI-X are based on a bus architecture, which means all devices on the bus must share the bandwidth. PCI is capped at 133MB/s, where PCI-X, which serves SCSI cards and high-end Network adapters, varies depending on the bus speed. AGP is based off PCI, but is a separate bus set aside to handle the demands that a video card has.
PCIe isn't a bus, so it doesn't need to worry about devices sharing the same bandwidth.
AGP support is getting pretty rare. Additionally, all newer AGP cards use a PCIe to AGP bridge, as ATi and nVidia no longer make their graphics chips to natively run with the AGP spec. These bridges tend to cause trouble, particularly on older chipsets.
I'm not sure what processor you have, but it seems as if you've got either an Athlon XP 2400+ or a Pentium 4 2.4GHz. In either case, any socket supporting these chips, Socket A or Socket 478, is dead in terms of upgrade support, so there is a cap on that. Additionally, all new motherboards use the 240-pin DDR2 memory standard, where the aformentioned sockets are typically paired with 184-pin DDR, or worse, 168-pin SDRAM, both of which are incompatible with DDR2 standards.
Upgrading largely depends on what you do now. If it's websurfing, you're probably fine as is. However, when it comes to gaming, you're going to be hurting. 512MB might sound good on a video card, but you need to understand that's part of some marketing: pair a low-end graphics chip with a lot of RAM and sell it. The amount of RAM is much less important than the chip on that card. Yes, it's not fair, but it's perfectly legal with the right wording.
