Windows XP works best with 2GB minimum, as it is right now you're probably running particularly slower using Virtual Memory i.e. the HDD for 'RAM'. So yes the system should be noticeably faster with 2GB.
As far as it being 'Worth It', IMO yes I'd get another 1GB matching stick or best 2X1GB set until you can build your new Ivy Bridge system.
ref - http://www.asrock.com/MB/overview.asp?Model=939N68PV-GLAN
- 4 x DDR DIMM slots
- Supports DDR400/333/266 non-ECC, un-buffered memory
- Max. capacity of system memory: 4GB*
Crucial -> http://www.crucial.com/store/listparts.aspx?model=939N68PV-GLAN&Cat=RAM