Eyefinity and crossifre doesnt work with the 5870's yet. It does work with the 5970 though so maybe consider that as its cheaper than 2 x 5870's anyway. You can't connect all 3 screens by displayport because the cards only have 1 displayport connection on them. The other two monitors need to be connected using the 2 x DVI or 1 x DVI 1 x HDMI combination.
If you get two cards you can't connect monitors to each card because the timing is different all monitors need to connect to 1 card only for eyefinity. The 5870 six edition (not released yey) has 6 mini displayport connections so you could use that with adapters to connect 3 displayport monitors.
A good 24" monitor with displayport is
http://accessories.dell.com/sna/products/Monitors/productdetail.aspx?c=ca&l=en&s=corp&sku=467-4327.