You'll only need 1 bridge, and it's definitely a shame that so many manufacturers don't include one. Any HD4870 1GB's will work. Another (less expensive) option is a Radeon HD 4850 X2.
http://www.newegg.com/Product/Product.aspx...N82E16814102809They can be overclocked a bit and are of course (much) less expensive than a pair of HD 4870 1GBs. The HD4850X2 is a massive card though: you'll need a long chassis to accommodate the long card. That's actually quite a good price for one too.
Four high-resolution high-performance choices:
Two Radeon HD 4850-512's (512MB usable memory), $200
One Radeon HD 4850 X2 (1024MB usable memory), $210 AR
Two Radeon HD 4870-1024's (1024MB usable memory), $300
One Radeon HD 4870 X2 (1024MB usable memory), $350 AR
A pair of Radeon HD 4850's would be the least expensive route but as newer games come out, 512MB memory will lose out to 1024MB memory. So it's more futureproof to use a pair of 1GB cards than a pair of 512MB cards. Two cards has one distinct disadvantage: they require two PCI Express x16-length slots, and having a pair of them usually means that you need a total of 4 expansion slots. The HD 4850 X2 has two distinct disadvantages: it is a VERY long card (11.5") and heated air stays trapped inside the chassis. However, I'd say the price is definitely right. And the last choice, the HD 4870 X2, is simply the most expensive. If I was buying one of those four, I'd be limited to HD 4850 X2 and HD 4870 X2 (due to having only one PCI Express x16 slot) and I'd probably choose the HD 4850 X2. I'm actually going to suggest that for you, and you can actually step back down to a micro-ATX motherboard (and I'd also suggest a micro-ATX chassis. There are some nice ones that can definitely fit a super-long card. If you want to use a pair of HD 4870s or a HD 4870 X2, I'd suggest you also look at stepping up to a Corsair TX 750 power supply. 100W more won't make THAT much of a difference, but it's always nice to have headroom and the larger (14cm) fan.
Cooler Master Elite 341, $40
Foxconn TW-080, $46
Antec Mini P180, $80
Antec NSK3480, $92
SIlverstone TJ08-B, $100
Any of those should fit a HD 4850 X2 in there just fine.
On one last note--a HD4850 X2 would last about as long as a pair of HD 4870s or actually any other cards from this generation. For gaming, that'd be about 2-3 years before you'd have to use Low settings at your native resolution. For the price, I'd say it's worthwhile.