I agree with those who've suggested going light on the treasure for awhile. However, I would advise against doing things to take away all their swanky items. At this point it would be a pretty obvious ploy to regain balance, and that might suck some fun from your players that they found all these cool things and now the're being taken away. Here's what I suggest.
Send your players on a dungeon crawl. Stock the dungeon with a goodly amount of no treasure beasties, like abberations and nasty vermin. Think monsters instead of villains. To keep your players from getting antsy, include a few pretty trinkets here and there, but of course they shouldn't be worth all that much, such as piles of silver coins, gems and jewelry. Stuff that has value, but not as much value as items, and that they'll have to go through some effort to convert into items. Keep the adventure fast and exciting, with a strong goal and promise of a reward at the end of the dungeon. (which, of course, should have some hefty story value but low monetary value.) Perhaps your kidnapped wizard is trapped down there?
The goal here is, of course, to provide your players with exciting adventures that they'll enjoy, while keeping the treasure down until they catch up to their wealth level-wise. The CRs I think should range from 6-their level, to 10-their wealth level. (based on the lovely math others have done in this thread.) I'd think quanitity rather than really hard encounters. The nice thing about these higher level encounters is that the party will level faster, so the gap will be closed more quickly.
Some other ideas for level appropriate challenges that won't be blown away by all the gear are traps (go by saves instead of attacks) Touch attack critters, puzzles (puzzles are such fun! and that's ALL brainpower there) and don't forget, plenty of roleplaying encounters.
Don't forget, you don't want to catch them up to where they should be wealth wise and then go all crazy with the items again. Keep it modest. Also don't forget, People have pointed out how ELs are different than CRs and aren't a 1 for 1 deal, but XP is. 8 CR 1s is an EL 7, but you still award XP of 8 x CR 1 for a 6th level party. That was one of the things that confused me verily.
I think that's it. Happy gaming!