It's really not worth it for Crossfire 512MB's.
When you crossfire, it's usually for higher-size monitors. When using a high resolution, you'll need planty of Video-Memory (VRAM). When one crossfires 2x 512MB cards, one gets 512Mb. The VRAM doesn't add up. PCI-E2.1 and 2.0 are perfectly complient with eachother, and will not notice speed reductions.
Also worthy of mention is that your Processor could do with a modest overclock. 666mhz if you can manage it will run perfectly with your 2x HD5770's.