Leaving the Internet Mail provider installed and running won't overly tax anything other than its minimal RAM use as it starts, there will be no activity as if there is no smtp (sending) email traffic it will have nothing to do but sit and wait.
If there is an attachment in the email that you want to download then gmail should have pre-scanned it minimising the risk and the standard shield should also cut you some slack depending on the file type being downloaded. You can get infected just by reading an email, there is a powerful function called an iframe tag.
This is an HTML code function that is usually used on web pages to provide dynamic content, e.g. import data into the iframe, this can just as easily be in an email (often used to import adverts) and in stead of importing data/ads, etc. it could just as easily execute a file on the internet. So I would hope that gmail would also have that covered, if not the web shield might also give some cover, provided the web page you are browsing your email over isn't https (secure).
And as Alan said to be viewed on your browser it has to be in the browser cache on 'your' system, be careful out there.