The sticky at the top of this forum section has answers to all your questions. But in the spirit of good will ill tell you the parts you need to know!
The two cards you use have to be the same model, so your other one will need to be a Radeon hd 2400 pro (not sure if the 'pro' is necessary or if a standard hd 2400 would work too). Also, its recommended that the memory of the cards is the same, but if you do use two with different memories the faster one will simply slow itself down to level with the slower one.
As long as the cards are the same model they will work, however the second problem you may face is your motherboard, i know from experience (bought my crossfire system last night!) that the P35 motherboards arent too good for crossfire as the PCI-E plugs arent both 16x, one is only 4x meaning the second graphics card you use will only be able to run at a quarter of its capacity. If however you have an X38 or similar mobo then both PCI-E slots will be 16x so it will be full speed.