For the record, the HD 5770 isn't an "enthusiast" card- more "Mainstream", which was enthusiast 1-2 years ago. Similar performance to the HD 4870 1Gb/ GTX 260.
Having said that, crossfired HD 5770's probably offer the best-bang-buck in graphics at the moment. Comparable performance to the HD 5870 up to 1920x1080 (or 1920x1200)- often with better minimum framerates, although the HD 5870 pulls better framerates at 2560x1600 or above.
I'm in the process of converting my secondary rig to crossfired 5770's at the moment.
As for the x16/x8/x4 debate, there are a few articles online to choose from- just make sure the article is dealing with the PCIe 2.0 specification which offers double the bandwidth of the previous 1.0 spec and has been standard on Intel chipsets since the P45/X38.
Here's a fairly good article from
Tom's Hardware (linked to the first results page but the whole article is well worth your time reading)
Basically, unless the card is dual-GPU/enthusiast class, i.e. pushing a LOT of pixels at very high framerates, then for the most part, the difference is negligable. A lot depends (obviously)upon how much the the GPU and the CPU need to "talk" to each other to render the graphics. If the CPU-GPU traffic flow is very high (usually down to how games are coded) then this can impact on the lower bandwidth (4Gb/sec) offered by a x4 slot.
EDIT
@Captain
mechanical drives must be nearing their limits- there's just so much you can do with moving parts.
These on the other hand I think will become the norm once we get past the "new technology" phase (ie.high pricing, shotgun approach to standardization, beta testing/early adoption, volume production).The MTBF promises enough going forward for hard drive manufacturers to look at RMA/DOA's of conventional harddrives as potential millstones.