290 Crosfire x16/x4 PCIe 2.0?

I maybe crazy but with R9 290s so cheap right now the idea of picking up another one is pretty tempting.  My only issue really is likely the fact my mother board is PCIe 2.0 (scouring the internet sounds as if it isn't as big of a deal) with also only x16/x4 mode for Crossfire.  My question is then would this be enough to not see a horrendous drop in performance, i.e. greater than 10%?.

Also the CPU at the helm would be a Phenom 2 1090t at 4.0 GHz (can't get it any higher sucks). Also for reference my MB is an ASUS M5A97 EVO AM3+ board. 

Another relevant piece of information is the resolution I play at is only a single 1080p monitor so I am not pushing loads of bandwidth with crazy resolutions.

The bandwidth would limit you. But your CPU would bottleneck it even worse. 

If you're only playing at 1080p a 290 will be more than enough. You don't need two of them. 

I think one 290 is enough for 1080p, unless you are running crazy downsampling. If you have a good PSU and the money go for it, but I guess you won't benefit much(at 1080p, because CPU will limit some games).

dude

I have dual 290's and I know I am bottlenecking mine on most games

you just need to make the choice being fully aware that it wont be until bare metal api's become more common that you would get your monies worth out of them.