No difference between any of those cards as far as memory and Graphics Processor Unit goes - they DO have different heatsinks though, which means in theory they have different overclock potential (to keep them cool while running them faster than normal). So no, not a huge difference - again, check the bundled games.
You don't have a PCI-Express slot. Guaranteed. Because you're current graphics card never ever came in a PCI-E version.
Unfortunately, it did come in a PCI version as well as the AGP version.
So the question really is - do you have an AGP slot? Which your current card may (or may not) be plugged into?
If the answer is no, you'll have to get another PCI graphics card or upgrade the motherboard.
If the answer is yes, then you can have an AGP one. They're just as fast as current PCI-Express ones anyway. Won't be true in the future (PCI-E can go a lot faster) but current cards don't really stress PCI-E so you're not missing anything.
Now, how to check if you have an AGP slot - have a look inside. There should be a few white connectors lined up - these are your PCI slots (pretty much all PCs have them these days).
If there's a brown slot, in line but set back a bit from them, that's an AGP slot. Yay.
Of course, it might be an old fashioned X1 or X2, rather than X4 or X8 (either of later 2 are what you need)...
...please just tell us what mobo you have. Look, here's everest to tell you;-
http://www.lavalys.com/products/overview.php?pid=1&lang=en
Or CPU-Z is an alternative;-
http://www.cpuid.org/
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