I have said it before, and I shall say it again. If one is able to afford a dual core, there is no reason what so ever to get a single core. With the exception of benchmarks, you will not see any difference at all in anything you listed going from a 2Ghz 512k AMD64(Let alone 2.2Ghz) to a 2.4Ghz 1MB AMD64.
Definitely get the dual core if you are going to upgrade at all. With a faster single core, your performance will go in games from 100fps to around 110 or so. Definitely not noticeable or even worth anything. In games that take a ton of graphical power, your cpu upgrade will have done nothing at all. With a dual core, you gain the ability to truly multi task.
With a single core, if you try and run an anti virus, or burn a disc, while playing a game, your game will slow to a crawl. For this reason, many people neglect their scanning of viruses simply because they do not want their pc to be relatively unusable for a period of time. With a dual core, if you burn a disc and play a game at the same time, the game will run just as fast running in parallel with the disc burning as it would if the game was the only program running. Disc burning and gaming are just examples. You can pick any type of system intensive task and swap it in and get the same analogy.
You also gain the benefit of being able to run multi threaded applications to their full potential.
Go for the dual core.
* One thing though. This only speaks for processor intensive scenarios. If a program is using your hard disks to the fullest, your game load times will suffer as a result. However, the in-game performance itself will remain excellent.
I have used dual processor systems and dual core systems for a long time now. If I was given the choice, I would take a dual core 1.8Ghz AMD64 system over a 3Ghz single core one. Dual core machines are just so snappy and responsive.