need help picking out a usb pci that fits my pc

im currently short on usb ports on my pc, and i want to upgrade. i saw one with 8 ports and one with 6 ports, but idk if they fit. also, they say you need everything up to vista, nothing about 7 or 8. im going to get this case
Antec Nine Hundred Case | eBay
im not sure what i have currently, but it says pavillion p6000 on the bottom. id also like to know if this case is better than what i currently have. any suggestions for a pci slot that has a lot of usb ports thats under $20? and a case that would be better than this one for the same or cheaper price? thanks in advance.

As far as I know there are only two sizes of PC cards, full size and low profile. Low profile cards are I believe half hight. So unless you have a mini itx case a regular size card should fit. To plug the card in you'll need a matching slot, PCI or PCIe depending on what the card wants/is.

ok so will this card Amazon.com: NEW 6 PORT NEC USB 2.0 HUB HIGH SPEED 480MB PCI CARD: Computers & Accessories fit in my case? i have a stock motherboard

Yes that should fit in a standard PCI slot. Assuming you have a free unused PCI slot on your motherbord.

how do i check? i havent added anything to it except 2 sticks of ram and a graphics card

You open the case and look inside for an empty PCI slot.