There is no 90nm 2800+ so that sort of eliminates that option but most people with a good aftermarket heatsink are reaching anywhere from 2.4 to 2.6 on air with the 3000+. So they are getting from 600 to 800mghrtz overclocks! Remember only the 939 chips (obviously not all of them) are made on 90nm currently (3000+, 3200+ and 3500+) (the 3500 is made on both 130nm and 90nm so you have to compare codes) (The 3800+, 4000+ and FX-53/55 are all made on 130nm)
You must check for motherboard compatibility with the OCZ DDR booster but it will work as advertised.
There is not a huge selection of 939 boards at the moment but the Nforce3 ultra would be a way I would go right now. Look at the websites and manuals and make sure you get good voltage options.
A good place for some pricechecks is
www.pricewatch.com