Do you work? An option if you do is to find a holiday, afterschool or childminder even who can have the children for a couple of hours and give you space and time. If you work you can get up to 80% of the cost back on CTC. Don't be afraid to use it not only while you're working but also when you need time on your own to get things sorted.
My house is exactly the same. And it used to stress me out big time. Especially when i would help my son tidy his room and then when i went in there the next day to clean it it was back to chaos. I told my son if he sorted his room out sufficiently, and kept his toys and dvd's tody in the lounge too that he could then have a friend over. So he had something to aim for.
So his friend came over, and when i went in there after his friend left the room was a zillion times worse than before! So i told him very calmly and while we were sitting quietly on the sofa how very diasspointed i was, and that unfortunately there would be no DS and no tv except of my choosing until it was sorted out. That is was too late to do it now but i expected him to do it in teh morning. He came and asked me the next day if he could have his DS... no... Can i watch tv... no And I reminded him of our conversation. He had a bit of a tantrum, went off to play in his room, and then came and asked me a little while later as it was his favourite tv show time. Again i said the same thing, and he shot up teh stars and started picking things up off his floor.
Much easier to do with one child i know. But if you get them all involved and say no tv until all teh rooms are sorted they will 'encourage' each other.
Do they have space to tidy all their things? Don't ask for everything to be where you'd like it to be, as long as you have a clear floor that's a start.
Go to your local pound shop and get some of those great big plastic storage boxes (with lids so you can stack them, and clear so they can see what's in them without emptying them out). Get one for you too. Get the children to throw everything in those. The room might still be untidy but the floor should be cleared. Give them a weekend to get their room sorted and anything left lying on the floor will go away in the box you got for yourself and they won't get it back till they show they can keep their things in order.
Reward them when they do keep their room tidy, gold star chart etc, choose something relevant to their age. When they all get 10 or 20 stars they get to choose a reward (list the options on the bottom), so maybe pizza hut, or cinema, they get to choose the dvd that evening, or a couple of £'s maybe.
My house isn't perfect and it never will be. And talking to friends i find that their houses are the same and they're in teh same boat as me. But getting it bearable with happy children and a less stressed parent is the thing to aim at i feel.
Try and take a time out when it gets too much for you. I've found speaking calmly and expressing deep disappointment gets more of a result than loosing my rag and ordering a clean up or else.
Hopefully others will be able to share with you their ideas, and you'll see that we all live in a state of chaos most of the time and find a solution that works for you.
best of luck
Gilly