So I got my Ibanez RGA8 today. I love the guitar but I've known for a while the pickups are pretty terrible. I'm looking at replacing them with EMG808s but I was wondering if I could maybe just put a single EMG808 in the bridge rather than having to buy 2?

If you wanted just the one pickup, you can do that easily enough. Just follow the wiring diagram. If you wanted one EMG and a stock pickup... you'll probably have problems. EMGs run on 9v batteries (18v if you want) and I think the stock pickups in the RGA8 run on a lower voltage. You might need to look into that if you wanted to mix them. (New EMGs also have a quick-connect system which doesn't require soldering, whereas I think the stock RGA8 pickups do...)

Other than that, I'd say look at the X series pickups, too. Listen to some sound samples, and see if you can hear the difference and which one you prefer more. You might prefer the normal EMGs. Personally, I swapped out my 808s for 808Xs.
Ibanez RG2228 w/ EMG808Xs | Line 6 POD HD500 | Mackie HD1221