There's no point mining with that desktop either, take a look at similar hardware in the mining hardware comparison posted earlier, you'd be lucky to pull 2Mhash/s with the GeForce 9200. You might do a little better with the CPU, but you won't even make back what the extra electricity costs you.

Why don't you install a miner, sign up to a pool and see what hashrate you actually get? It'll only cost you a bit of time, the energy cost will be tiny if you only run it for an hour or two and you'll learn a fair bit along the way.

If you're still keen after that, brand new Radeon 5850's are still available in Australia for around AUD170 that will give you 350 to 400 Mhash/s. That would be a pretty decent upgrade to your desktop, and would likely still be marginally profitable even at Australian electricity prices (say ~AUD0.22/KWh if you're on an average plan).