|Well, given the specs on the link you provided (Acer Aspire AM3100-U3401A), you have one PCI Express ("PCI-E" or "PCIe") x16 slot.|
This means just about any modern video card can go in that computer. You can shop around yourself on computer part sites (for example newegg.com, tigerdirect.ca) for PCI-E x16 video cards to get a good idea of the types of cards you'll find...features they offer, variations in specs, etc.
RE: your question about compatibility.
In the vast number of cases, your integrated video will automatically be disabled when you install a new video card (so you would use the monitor connector on the new card, not the one on the motherboard).
If you're planning to buy your card online, you can probably find a bunch of cards in the $100 (US or CA, doesn't make much difference) range. In major chain stores, you're a lot more limited I think. Try local "small" shops which custom build computers...you may be able to find a really good deal, and in some cases might even find installation included in case you're a little wary about doing it yourself.
Edit: I believe this should have been posted in General Hardware. Keep it in mind for next time :)