Sir P
I believe the (lack of) Memory is your problem.
I've been a long-time Desert Combat player, and over time my system upgrades have made a noticeable difference in performance. I found that with BF1942 / Desert Combat, upgrading from 512 to 1 gig or ram was the single biggest boost for the game - faster load times, smoother graphics, Double plus good.
Playing the demo all weekend long, has made me realize that I am going to order a second GB of ram along with the full version of BF2, While I have had a few laggy moments here and there, I would say that BF2 is a huge improvement over BF1942 in the network department.
Overall, I am very pleased with the demo, and I was very worried that it was going to be a letdown ala Unreal Tournament 200X - where everything looks real purty, but the gameplay goes down the tubes.