I would get an AM2+ mobo for phenom, there have been problems with some of the am2 ones supported via bios update. I would not bother with old N Force 570 / 590 boards.
A new range of AM2+ NVidia chipsets are on the way, designed for phenom. If you must have phenom & SLI, I would wait for these. The midrange one should suit you fine & be well under £100. So far only the non-sli with onboard graphics are out from the new nvidia range.
The SLI ones will be called 780a SLI & 750a SLI respectively, the former being high end (prob over £100) & the latter mid range (prob less than £100)