is my memory usage high?

As long as you don't start paging (using more virtual memory than you have physical memory), you should be fine.

What metric are you using to decide that a lot of memory is being used? The free memory counter being low and Cached/Available being high is expected.

Using the Task Manager - but actually its pretty obvious when "swapping" starts happening, your machine will slow to a crawl and the hard disk activity light will be flashing on and on. Swapping happens when you have more programs competing for system memory than you have available, so the OS has to keep copying application and data between physical memory and hard disk (the paging file).

Hopefully you get close to over 90% memory utilization with most of it allocated to replaceable functions such as disk cache or virtual memory and not to currently executing processes.

I think your mem usage sounds acceptable. One thing, make sure mediabrowser is up to date. The most recent patch fixed some memory leaks associated with the service component. The leaking wasn't terrible but if you go extended periods of time without rebooting (like me!) it will add up.

No-but you will want to look at your hard drive activity which will likely be your bottleneck. If your filesystem become fragmented or you have too much IO and the system crawls to a snails pace then you will see substantial degradation in performance of AV content playback/recording.

Antivirus programs are notorious for being resource hogs and hook the filesystem and networking protocols so that they can inspect every bit coming/going on the machine and scan/quarantine it if necessary.

Most things are IO bound nowadays, not memory/CPU limited.

Lightweight AV, use Microsoft's Security Essentials. Also to let you know when programs are out of date and needing updating, use Secunia's PSI http://secunia.com/vulnerability_scanning/personal/ As for the memory use, keep in mind, that Windows reserves a certain amount of RAM to keep in reserve. I am in no way related to the program writer of Meminfo, but I have copied and pasted what majorgeeks states about the program. There are some others over on the site, but I just picked this one as the first one that I found.

Meminfo display the current memory (RAM or Page File) usage of Windows in the system tray.

With a small gauge graphic and the value updated in real time, MemInfo gives a fast feedback to current memory usage in Windows.

MemInfo is highly customizable; Choose when MemInfo should warn you about high memory usage, show physical memory or Windows page file, show free or used memory, choose the font that looks better. MemInfo cames with a built in memory defragmenter to allow speed up your computer by maximizing the available free memory.http://majorgeeks.com/MemInfo_d5308.html