I'd also recommend that you consider the Alesis DM Pro. Its not as inexpensive as the other suggestions, but its programmable as all get out, has 2 FX processors onboard, can layer up to 4 patches and the number of available sounds is huge. There are a couple of earlier Alesis models as well. You might want to check out the 2nd-hand sources such as ugbm.com.
Check with Yamaha for specific loop disks/CDs, too. They're not always forthcoming with a lot of such data, but they have their moments. Likewise drum CDs or WAV/AIFF files via the Net. You can always load a WAV file and send the note data TO your drum box, which could lead to some interesting kit assignments. Between your RX5 and Acid, you could get some massive action going. When you share the workload between 2 or more units, your options grow exponentially, although your headaches can do likewise!