If Gmail were integrated with Google Docs & Spreadsheets, it would have a new link next to some attachments that says "Import in Google Docs & Spreadsheets". This way, you could edit these files online.
Fortunately, if you go to this page, you can find a mail address where you can send your documents to be uploaded in Google Docs. So you can add that address as a contact (let's say Google Docs), and forward the mails that have .doc or .xls attachments to that address.
You can even add a filter that sends all your .doc or .xls attachments to Google Docs & Spreadsheets:
Has the words: doc OR xls
Forward it to: (the email address discussed above)
The filter will work only for messages received after setting the filter.
For the moment, you can upload only files smaller than 500 KB and you can't upload spreadsheets by mail (but this feature will be added soon).