NOOB question - My motherboard does not have a PCI-Express slot, only PCI. Will a PCI-Express card work in this slot?
Even though they sound alike, and indeed, the slots may even LOOK a little alike, rest assured that they are COMPLETELY incompatible.
While thye share a similar array of "pins" in the center, it's the way they USE them that's different.
To put it in short, PCI is a "parallel" interface, meaning that it uses ALL of the pins for each signal pulse; using them, it can transfer up to 32 bits at a time. Given that PCI operates at 33MHz, that yields a potential maximum transfer rate of 133 MB a second.
PCI-e, in spite of the name, is a "serial" interface, meaning that it uses multiple independent channels; (called "lanes" here) for a PCI-e graphics card, there are 16 of them: hence, that "PCI-e x16" or "PCI-e x1" names you might've seen listed the numberl of lanes that slot had. Each lane can only transfer one single BIT at once, but they can then run at absurdly high speeds; I believe it's 2,500 MHz (2.5GHz) for PCI-e. Because some "overhead" (error correction, etc.) are figured in, it works out to an effective 2,000 MHz, for 250MB a second PER LANE, or a total of about 4GB per second for a PCI-e x16 graphics card.
As you would now realize,it's technically impossible to get one to work with the other, and, unless I did a bad job of explaining, you might understand why.
Now, back toward the REASON for your question... You have no PCI-e slots. However, might you have an AGP slot? Those may technically be "outdated," but are QUITE usable for even today's high-end gaming. Cards are more expensive for them, though. Note that a PC can have only one AGP slot; it will USUALLY be tan or brown, and ALWAYS be the one located closest to the CPU.
If you don't have an AGP slot either, then you're in trouble. there ARE graphics cards for ordinary PCI still around, but they are over-priced, and simply don't match today's cards in any way. Your best bet would be some nVidia GeForce FX cards; the 5200, 5500, and 5700LE are still not TOO hard to find for PCI. There was also the FX 5600, but that was discontinued long ago. I've also heard suggestions that there's a PCI version of the GeForce 6200; if it does indeed exist, that would be your best bet, hands-down.
However, with only PCI graphics, don't expect TOO MUCH. I've personally gone quite far with using some of my PCI graphics, and even played some more recent games, but they would never come close to just about any of my AGP cards.