I currently ONLY use the 300x250 because long ago when I ran two units I found that my epc dropped quite a bit. However time has gone by and i'm considering adding a 160x600 to the right side. Anyone got any opinions on keeping one ad unit so the most expensive ads show vs using two in hopes of improving CRT or overall revenue?
You can use upto 3, so the main points you should consider is how you want your page to look natural or full of ads? If you are getting enough clicks (CTR), you are okay with 1 or 2.
Words of google adsense team "Currently, AdSense publishers may place up to three AdSense for content units on one webpage. You may also place a maximum of three link units and two search boxes on each webpage." visit below link for more detail https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=9735
Lol....quite impressive how everyone so far is missing the point: Running too many ad units on the same page can dillute the ads that actually show up, meaning that sometimes you can get much higher CPC by running fewer ads. OP should A/B test the implementation of an additional ad, since that's the only way he can reach solid conclusions. It's impossible to guess such a thing. Split testing your best friend, when trying to figure out which placements work best in any website.