Are you sure other than the IE, songs and msn there  is no background software running? Which monitor software that you use to monitor the cpu usage?? I'm running on P8400 and HD3450 and I never face any issue with the lagging or hang. I'm running FF with 8 tabs at least and gtalk+msn. On top of that, I'm will run photoshop or after effect and yet, the performance is still great. My laptop get a lil sluggish when I converting vids to iPod format with HQ setting but other than that, I have no problem whatsoever. I'm not talking as a Dell fan, I'm sharing in term of laptop performance.

P8600 is one of the best bargain cpu that available in mainstream laptop market and it's performance proven by some users. And I beg to differ on HD4570, that is another best bargain gpu on mainstream market. I don't know how you configure your laptop but maybe you should check certain aspect to improve performance. For example, which power option that you running on and did you do frequent housekeeping on your HDD?
Just called and tell them your touchpad got problem. Regardless what happen during the conversation with the customer service, just tell them got problem and ask them to fix it.

Another quick solution, external mouse
Have you tried surveying Bagman or Crumpler sleeves?

I believe there will be a sleeve for the 15.6' laptop as currently the 15' laptop in the market are majority 15.6'. You should bring along your laptop when shopping for sleeve, you can try it right away. I saw few sleeves that fit 15.6' but those are branded stuff, ain''t cheap for a sleeve.