I have said a few words about our apache performance before. You could see the result, that it may have taken a while until the http request was served if it was very crowded. In the end, that also did not help anymore and the performance was very poor. Actually, I have been spammed with mails from our monitoring system telling that the server load is very high.
Finally, we have moved to our new server last night. It smoothly serves the requests now and still leaves room for more requests.
I hope that this will do for a while and what to thank everyone for the donations for our server expenses!
Update (2010-11-21): It just does not work as you want... After bringing pdfforge.org online, the XenServer got very instable and froze every few hours. After several days of random outages and tests we have decided to downgrade to XenServer 5.5 which seems to run well but has other problems with our Ubuntu servers. So we are up again but there is much space left for optimization.
The good news are that the new server really has much potential. We are able to server far more requests than we have at the moment which leaves enough space for many more users!