Proxy, Web and Exchange services all vary widely in their resource utilization. For example, if you have server static HTML, your Web server will incur minimal processing overhead and will be bandwidth-limited. If you have complex dynamic pages, it may very well be processor limited. FTP is always bandwidth limited.
You mention you have a proxy server. Is this being used as a firewall? If so, I would make Server No. 2 the proxy server and not put any other services on that system. This will minimize the potential for vulnerabilities on that system. Server No. 1 has more memory than Server No. 2, so it can better handle Exchange, Web and FTP services. As noted above, you should make sure you're not going to max out system resources, but this depends on your utilization patterns.
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