If this is your computerhttp://www.shopping.hp.com/product/rts_desktop/rts_desktop//3/storefronts/NY552AA%2523ABA
Then A 4890 would be a good choice to put in there since it's a pretty descent system on its own, but there is no info on the power supply. You can open up the case and look at a sticker that is ussually on the psu, but I'm gonna assume that its an oem 350w which wont run a 4890, so I would look at buying this psu. http://www.newegg.com/Product/Product.aspx?Item=N82E16817341022
It's cheap after the mail-in-rebate, it is semi-modular so you will save allot of space in the small case and it'll run any single graphic card. If I were you I'd wait till the 5830 is released. Apprently it performs likes a 4890 with all the 5xxx series features with a low price.