Have a look at
to get some ideas about what's out there.
If you're only going for a motherboard, the usual suspects are Abit, ASUS and SOYO. for £160, you should be able to get a top quality board.
If you want to get a motherboard AND processor, then, as paparazzi says, the Duron 850 is a good, cheap choice. You should be able to pick up an Abit KT7A cheaply as well - it's not the most up-to-date, but few would dispute its place as a classic. It should also fit your budget, and allow you to re-use all the parts you already have.
If you go this route, you should upgrade your PSU to a 300W model, and consider extra fans for your case. AMD chips love to be kept cool.
If you're in the UK, GSquared will do a reasonable deal - and provide superb support (I'm not affiliated to them, but they're local to me, and I rarely buy my components anywhere else).
Their price list is here
but to save time, their price for a KT7A board is £95 (or £127 for the RAID board - recommended), and a Duron 850 is £49 - so this fits your budget nicely.
I hope this helps