Ew, I hate criticising, so lets assume your model is un-finnished, ok?! I think you realy need to double up on the linear bearings, four per axis would be a lot more stable, and the increased width of the frame, both in x and y, would help your deflection problem. A cross piece between those gantry sides would also help stiffen it up nicely. Go and dig out Jeffs (HomeCNC) router thread, and take a look at his structure. If you're going to buy ballscrews, it's worth having a decent frame to mount them on, or you will not gain anything over using all-thread.
Sorry!
