How do I know if I can upgrade my computer?

As you may know, I was going to buy a dell, but I was given this computer. I'm not sure if it's able to upgrade though. How can I tell? I want to upgrade the memory and put a video card in it.How do I know if I can upgrade my computer?
The easiest way is to either look inside the comp and see if it has an AGP slot or call Dell and give them your pc number and ask them if it has a PCI-E slot or a AGP slot.. How do I know if I can upgrade my computer?
I don't have a dell. Actually, I don't know which brand it is. I will open my computer tommorow. What should I look for to know if I can upgrade my memory and if I can put in a video card? I heard of PCI but that it's outdated and I should go for PCI-E or AGP. Maybe I have AGP in my computer. Where should I look at?EDIT: Well, I have an P4BGL-MX. It has no agp slots or PCI-E. Only 3x PCI Slots. Is there any way to put in an AGP slot? or are there a couple of good pci slots still out there that can run the games like counter strike source and grand theft auto san andreas? im not a big pc gamer, so i just need a card that would run games that came out about two years ago. maybe halo 2, :D.
bumpis this a good pci card? i read the reviews and it says they can run half life 2 on med settings.http://www.newegg.com/Product/Product.aspx?Item=N82E16814133007
just get a AGP and look on google for the instructions on how you can cut it so it fits in an PCI slot. ;) I am only kidding if your limited to PCI then there is no real upgrade you can do. your only choice would be to start from scratch but dont worry a hole new rig only costs $800 (only lmao). about a month ago I managed to build a kick @$$ rig on newegg for $800 (2 gigs of ram, 8800 GTS. etc etc) only problem is they dont ship to Canada :(
oh come on man. is that the best help you can give me? lol, are you sure any pci card might be good enough for one year?
hahah sorry for some reason i thought u said uhad a dell.. If you want to play Half life 2 get a ATI 9550 pci card my bro got one and he plays Half life 2find withit.. But makes sure CPU is decent and that u got atleast 512mb's of ram..
thanks, and i have been making a couple of topics of thinking of buying a dell but i will use the one i have for a while then build one. thanks.