Opened exactly the same apps as you did. At the beginning, after just Bounce, PDF and web, it already slowed down a bit. But a few secs later it just ran all of your apps pretty smoothly. So after that I opened up App.Manager and Angry Birds(pretty heavy game). This put the N900 into a very annoying condition and it wasn't running smooth anymore at all. But also this decrased after a while. So i decided to open up more apps. I had all your apps, App.Mngr(downloading), Angry Birds, Witter(twitter client), Irreco, the phone, foto browser, calendar and gBoggle(a small game). That were 16 apps. And I was online on msn as well. Everything was working, in essence, but it was horrible. I finally got a 'web close-page' popup and the phone was going realy very slow.
After this when I accidently opened ATI Calculator, the N900 froze.