I currently have a MSI HD 7850 2GB card.
I have a question as to if I should by a 7870, or get a second 7850 for crossfire, which way would yield better performance?
By using the 7870 you will see an increase of 10 to 20 fps (usually less than 15) in most games...but with a crossfire you will get WAY more performance. In some games ,you can even achieve a 80% fps boost by crossfiring two cards (e.g. Trine 2) ,which would mean almost double fps in theory... so I would definitely recommend crossfiring ,instead of wasting the money on another card ,which will only bring a minor improvement...
Ofc make sure that your PSU can handle that extra card.