Really the next step up from imovie is Final cut express, a very good bit of kit, but that will set you back $200 unless you get a bargain. You could also look into adobe premiere elements but I'm not sure how great that is compared to proper adobe software (you can download a trial version i think)
If you're frustrated with the new iMovie, you can download the older iMovie free, which is a bit more serious for editing. You can also get plug-ins like http://store.apple.com/us/product/TK710LL/A that extend its abilities, but I haven't tried those.