are you using a 64-bit OS? you'll only have 4GB available with win32. there are 64-bit editions of xp, vista, and 7 available though. 64-bit OS and "4+ core" is a migration you can't put off much longer so you may as well get them out of the way.
back to the CPU, the Athlon 5200 and 6000 will both provide a nice boost to your singlethreaded performance, but its likely that those poker table applets all run within separate processes and a quad core might do a lot more for you than a higher-frequency dual core. I agree with shawn that you should get your quad upgrade out of the way now, and I also agree that you want a little bit higher frequency than just 2.0 GHz. it's stupid to upgrade from a slow dual core to a fast dual core, but also stupid to go from a slow dual core to a slow quad. and you certainly don't need a new motherboard to get a quad. ASUS's website explicitly states that your motherboard supports the 65nm Phenoms (might be wise to update to the latest BIOS before you upgrade, though).
Looking at newegg, you can get 2.3, 2.4, and 2.6 GHz Agenas for 110, 120, and 130 dollars respectively. They would improve your single-threaded performance due to their higher frequency and improved cache and more than double your multithreaded/multitasking performance which your usage patterns seem to reflect. if you use firefox or chrome, each poker table will certainly have its own browser process, so that not all processes will be waiting in line on the same core. This is a 125 watt processor, however, so make sure you have decent airflow in your case. Despite the phenom's crude 65nm heritage, people were still getting 200-400 MHz overclocks with a slight undervolt. something to consider.