The 3800+ cpu, is this a single core cpu you have? If it is then theres no sense in you buying the Athlon 64 4000+ since the difference is only the 1MB L2 cache on the 4000+ as oppose to 512kb L2 cache on the 3800+
You should try and get a use 939 Dual Core CPU, most new games coming out site the 4400+ as the recommended spec. I presonally feel my 4400+ is a little long in the tooth with my current build, but it sure beats any single core.
Fish around in the FS threads for used X2 on 939. That should give your system a little more life.