Not sure about the only running on one stick thing, although I can't see how that could be a problem..
But having a quick look on Dabs shows up PC3200 Non ECC DDR ram (which is what you want for that board) single sticks of 512Mb are only about £25 - £30 each. Thats bloody cheap in my book..
I've now got a shitty Toshiba laptop that I upgraded from 1Gb to 2Gb and I think its worth it (my desktop is now running on 4Gb) and I can't recommend throwing a few quid at it enough..
Rather than spend the time and hassle of just doubling the size you have in there, i'd personally go for 2 sticks of 1Gb (which is the max your board will take) as you'll only end up being tempted to do it anyway (if you're anything like me..)
This is the cheapest 1Gb sticks I can see that suit your system on Dabs, you may be able to find cheaper, but in my experience you get it mighty quick and they don't fuck about if it doesn't work for any reason..
Hope that helps
"It'll be alright mate"