How do I make Windows 7 less aggressive in caching memory?

The Standby memory is not a problem, it is considered "free" and will be reallocated on request - it just contains memory recently used by programs, but is now freed. If another program requests more memory than is "Free" the system should allocate from the Standby set seamlessly (since "standby" memory isn't really in use at the moment).

Your excessive Modified memory is your problem, because it cannot be reallocated easily. See here. It's likely to be caused by either a memory leak (which seems to correspond with your first paragraph - what programs are leaking?) or a page-file being too small (is your page file fixed size or deactivated?).

The SuperFetch pre-emptive caching is not your problem. Windows 7 will use as much spare memory as it can to pre-emptively cache what it thinks you are going to need next. If it gets it right, it saves you a bit of time and your applications appear quicker and more responsive.

If it guesses wrong, and you need some other data, Superfetch will release some of the memory it has, to enable you to load the data you really need. See: http://arstechnica.com/microsoft/news/2010/02/behind-the-windows-7-memory-usage-scaremongering.ars

If you are getting 'low memory' problems, it isn't going to be because of SuperFetch - as it will relinquish as much memory as necessary if applications demand it. You available memory (550MB) is what is being used by superfetch, so clearly you haven't ran out of memory yet. Even when you do use up all your RAM, your machine will fall back onto virtual memory, though you will want to avoid this because of the performance hit...

So it suggests you are 'low memory' problems are due to problems elsewhere - perhaps one of your leaking apps is consuming (and not relinquishing) other types of resources...

Either way, the solution is more likely to be found by tackling your buggy applications, than by messing about with SuperFetch...