Saving a garden can be hard work, but if you are up to the task anyone can do it. First of all, how weedy is the space? It may be possible to pull the weeds out and not disturb the new perennials. Often people jump to a worst case senario first when some tenacious weeding would do the trick. If it is truely too weedy to do that, then I would remove the plants to pots where they could be cared for for about two weeks, and use a herbacide like roundup to clean up the whole space. If the garden is not edged, and you are having problems with grass, then I would install some kind of edging, contractor grade black poly edging helps reduce the grass invasion on the perimeter of the garden. Once the edging is in and the weeds are dead, you could retill it and use that opportunity to add some compost and or composted manure to ammend the soil. Once your soil is ready, then replant your potted perennials. I would make sure that I pulled the weeds out of the potted perennials as well, chances are (if the garden is a huge mess) that you will dig up some weeds as you dig out your plants. Mulch will help protect your garden from future invasions, there are many schools of thought on what kind of mulch to use, and each gardener has a personal preference. In the spring, you can use a preventative herbicide like Preen in the bed to help keep the weeds down. The cruel reality is that you need to tend the garden and pull the weeds as they come up. In the early spring when you have many tiny little weed seedlings, you can do this with a hoe (but you cannot disturb the top of the soil in that way once the Preen is down). Grass is a dilligent foe in the garden and you have to match its dilligence, pull pull pull...it will get easier with time.

As to the time frame, you could still do this this fall if you can get to it soon, otherwise spring will be fine.

Good luck...every one of us started out as new gardeners, the lessons we have learned by making mistakes are those we learned best!

Helen.

Some perennials actually prefer and thrive transplanting in the fall. Other plants, I think the more shrubby plants such as roses, would probably do better in the spring, especially in the more northern areas. If you decide to transplant in the fall, I would do it very soon (like this weekend if your ground is prepared). Water well. When the ground freezes to an inch or two, mulch well with shredded leaves or shredded straw. (Depending where you are, this could be the last week of October or way into November). Mark the plants well so you know where they are in the spring and can gradually uncover them as it warms. With good mulch, I have been pleasantly surprised. Without mulch and no snow cover, I have lost a lot of plants.

How many plants are you talking about? 10 or 10,000?
I would chalk your failure up as invaluable knowledge of what not to do in the future. I had to learn that too.
If me, I would wait til things die down then move what you have invested in time and money to a planting bed this fall.
In the Spring, when things are really crispy dry and in great fire-hazard state, the dried weed grass is ripe for burning. Light it off. Careful-don't burn down the county. 10 days later all the bad weed-stuff will be emerging green and you drown it with Roundup. Yes-poison it. The benefit of this is that all the good prairie stuff know better and will not emerge for some time yet, and they will survive quite well. Next summer you will have all your new plants plus the good wild things.
And that is the lesson I had to learn. I can show you pics if you want.