OK...good news and bad news.
Good news first: PCI-E is the bus you need to run modern graphics cards.
Bad news: Your options are EXTREMELY limited, mostly because of the form factor. Slimline computers are a PITA to get graphics cards for and you can almost never get the high end cards (they're too big for the form factor). Slimline cards also tend to be more expensive than their full size counterparts...
My ATI recommendation for you:
http://www.newegg.com/Product/Produc...82E16814102874
My nvidia recommendation:
http://www.newegg.com/Product/Produc...82E16814500177
Neither of these cards are great for gaming, but they're likely the best you're going to be able to do for the slimline form factor (and not to mention the $75 budget).
