I think this is a great and very useful thread.

I've switched between using my own cc's conversion and Paypal's conversion at times b/c at times you'll find there's actually a lag in Paypal's rates which can be to your benefit on days when the exchange rates are depreciating.

On days where there's high upside, I often opt for my cc's conversion. Most times I've gotten it right though I generally avoid weekend transactions b/c the lag before it shows on your cc statement is longer (sometimes you have to wait to Tues/Wed before it appears if you pay on Friday). I try to make payments at the top of the week for that reason.

I also use:
http://markets.theage.com.au/apps/mkt/forex.ac

as my source for rates. It's fairly accurate if you take the average of the two buy and sell rates.