Solid, supported SVG plugins for IE, AFAICT, have gone the way of the dodo. (You're not the only one clamoring for a solid SVG plugin for IE. You may have already found the Ssrc plugin and noticed it's 32-bit only.)
But you have quite a few options that aren't SVG plugins.
If you can get your users to do it, the easiest option would be to prompt IE users to install Google Chrome Frame (available for 32- and 64-bit OS's), a much more invasive plugin that supports SVG display along with all other Chrome rendering features.
If you can't count on Chrome Frame (you're not sure people will be comfortable installing it, or they won't have permissions, etc.), there are still ways to shim SVG output in IE6-8 (ordered from most recommended to least):
- Use SVGweb. Flash required, but most commonly used SVG specs are supported and performance will likely be excellent.
- Use excanvas.js. See this question. Might be a pain to configure and unforgiving, but excanvas is mature.
- Use svg2vml. I've never used it, and you'd need to connect it with processing somehow.
Another option would be to drop Processing and use Raphael, but I'm guessing Raphael's not powerful enough for your needs if you're using Processing.
HTH