PCI slots are usually white, AGP slots are usually brown. The one you need to look at is usually the one at the top of the board nearest the cpu and power supply. You'll have to open up the case to tell for sure, and you can also see if the computer manual has a section on upgrading the video card if you still have the manual. You may also be able to contact emachine's customer support and see if they have information on your specific computer's video card upgradability, but realize they might give you a generic and incomplete answer because sometimes the tech support people know less than you do about computers.
If you have a PCI slot, you can get a reasonably cheap PCI video card for about $50. Get either an Nvidia FX5200 pci or a GeForce4-MX PCI. Either should run under $80ish with the Geforce4-MX being cheaper.
If you have an AGP slot, you can get any AGP video card that your budget allows. In this case, avoid the geforce MX cards because you can get a lot better card for not much more money. An ATI radeon 9600, 9600pro, 9600XT, etc. should cost just a bit over $100 and will be pretty nice. An Nvidia 5700 card can also cost just a bit under $100 and would be nice too.
Shop around and see what other people have to say.
