I just punched the model number into image search and it's a standard case, and HP tends to use fairly standard parts. No proprietary garbage and such.
Of course you couldn't find the model number, it's not even from the same country. Even fry's and Best Buy have their own model series (the pavilion elites end in f in the fry's model code, y in the bby model code), so another country changes it up...a bit.
The PSU definitely poses a problem, as that's the sister series to the p6x series, all of which have 350 watts or less. More than likely you'll need a PSU upgrade for any decent card, but on that rig just about any modern PSU will work. I would recommend at least 700 watts (it will be high enough to use later on), and cable management never hurt! The good news is these are all regular boards, and you won't have many compatibility issues.
My recommendation would be to go old school and measure the space you have in the case (leaving an inch), and find 5-6 cards you like. Look at the specifications, and manually check the products (NOT the packaging) width and height. Use a reputable vendor though.
Good thing I can read Russian, but still no data that is 100% on the motherboard. I'm familiar with the sister series' (there are more than one of them), but I can't say for sure. Open the case up and take a picture?