Quality amps usually have a limiter on them to prevent you from overpowering your speakers. You should have at least 600 watts of power... if you are running a monitor you will probably need two amps. One for the mains and one for the vocal monitor (unless the monitor is self powered).
You need the power for reserve... not for volume. That way your lows are thumping, and your highs are crisp and not distorted. Thus a 600 watt amp is usually 300 watts per channel. Any more power and you'll blow people out of the bar.
Here's what I'm using:
ACESONIC AM-825
Look at the features of this baby, and I think you'll find it has everything a KJ could want and more. The sound quality is awesome, and with the speak on jacks, set up is a breeze. It has a pass through for your EQ or to add a sonic maximizer (although it has one built right in!!). I don't use an EQ with this at all... it has adjustments for lows, mids and highs, and with the BBE sonic maximizer I use an EQ would just get in the way.
If you use a laptop, it has a USB port for a direct digital audio connection (no digital to analog conversion!), and an S-video, composite video and RCA video jacks, as well as a VGA connection!!!!
I love this thing, and I looked all over the place, and could not find anything like it anywhere.... and it fits in a standard 19" rack!! Thus, you can prewire everything. Just plug in your speakers and your ready to go.
I didn't start out using a laptop, but now that I'm getting one I'm so glad I thought ahead an got this.
Acesonic is great to deal with, I've been using their karaoke player for over a year... also works great.
This amp also has a vocal line out for recording purposes. Just run the vocal line out to a second PA amp, and run the monitor from that. You don't need nearly as much power for the monitor. I'm using a 100 watt bridgable amp in mono. Works great.
Hope this helps!