Well i just saw some interesting stuff on the net.
I can actually run 2 different cards for example the 5870 will work real good with a 5970 since the 5970 is considered to be a 5870x2 basicly if i would have a setup like 5870+5970 It would be simply like running 3 HD5870's
I will need a new powersupply how ever... I might rig my old PSU to run on both Video cards and use another PSU to power the other components But thats just an idea ill probably buy a new 1000Watts PSU (not doing 850watts because that would stress the PSU to its max when using on long term)
Anyway You cant use a HD5870 with a HD5770 because they are not in the same 58xx series (the 5970 is an exception) You can only crossfire cards from above 56xx to the latest but they all have to be in the same series like 58xx, 59xx, 56xx, 57xx,
Just google crossfire 5870 + 5970 to get more information or just google crossfire.