|You haven't told us which mboard model you have installed theses cards on.|
CrossfireX mode is supported only for two or more ATI video chipsets on mboards that have certain AMD, or certain Intel, main chipsets. It's not supported on mboards that have NVidia main chipsets.
CrossfireX Combination chart:
If you DO have a main chipset on the mboard that supports CrossfireX, then that chart shows the HD 5750 and HD 5770 combo can be run in CrossfireX mode, without needing a cable to connect them together.
"I can't notice much of a performance increase in crossfire-compatible games."
What info are you basing that comment on ?
Just your own perception, or did you run a benchmark program of some sort, in the game itself, or wherever, before and after you installed the second card ?
If you haven't run a benchmark, then run one with one card installed, run it again with both installed.
If whatever game isn't performing any better according to your own perception, it may have been already running at or near it's max performance with one card installed.
"I checked to make sure crossfire was enabled in the CCC, it was, and GPU-Z says it's enabled on the first card (5770), but not the 5750."
CCC is likely to be correct - GPU-Z may not be.
I don't know what you're supposed to see in any case. Maybe in GPU-Z it's normal for one card to show it's in CrossfireX mode, and the other to not show it. CrossfireX mode can't work unless both cards video chipsets support it and can be a combo, so if one card shows it, I would think both must be using it.
You MAY need to change a setting in your bios Setup when you have two or more video chipsets on cards in CrossfireX mode - see your mboard manual - however, I would think Catalyst would be able to detect if a setting in the bios was not right for the situation.
I don't know if it makes a difference which of the two cards you connect the monitor to.
E.g. It MAY be that you have to connect it to the card Windows sees as the Primary display in order to actually see the effects of CrossfireX .
What is the max (combined total output) wattage capacity of your power supply ?
ATI Radeon™ HD 5750 Graphics
* 450 Watt or greater power supply with one 75W 6-pin PCI Express® power connectors recommended (600 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)
* Certified power supplies are recommended. Refer to http://support.amd.com/us/certified... for a list of Certified products
Same requirements for HD 5770 .
600 watt or greater PS max capacity recommended for two cards, or for two of the same video chipsets on one card
If you're a gamer...
In most if not all cases, the max capacity rating of the PS is an intermittent rating. It's recommended that you do not load your PS to any more that 80% of that rating if you are going to be using something that puts a constant load on it, such as playing a recent game for hours on end. In that case, you multiply the min capacity stated for the system with the particular video chipset on the card by 1.25 to find the min. capacity of the PS you should get.
In this case - 600 x 1.25 = 750 watts or more