What media front end are you using?
If it's XBMC then you're in luck! Well, you are if you're willing to do a little surgery and invest about $30 more.
You can purchase a Broadcom Crystal HD and open the mini up, pull the wifi card and install the Broadcom in the PCI-E slot.
XBMC's developers are promising the near release of OSX drivers to allow the use of the Broadcom to watch HD 720 and 1080p video and take the burden off of the CPU which will give your mini new legs!
Once again, this is only if you are: 1) patient (as they haven't released the drivers yet); 2) Willing to use XBMC for OSX (which is great, although I'm a PLEX user XBMC is still great); 3) Willing to crack that mini open and update it, and are able to have a wired internet connection to it.
Finally, if you are unable/unwilling to do the above, you could sell your mini on craigslist for probably between $300-$350 relatively easily and then invest that plus a little more in a current gen.
Let me know if you have any other questions!