I wrote the Mahalo guide for this game, so I pretty well played the heck out of it.
Here's the walkthrough:http://www.mahalo.com/Dead_space_walkthrough
But there are some tips for defeating Impossible mode that will make things easier:
1. Concentrate on as few weapons as possible. The game offers tons of guns, but really you don't need all of them. I found the flamethrower and pulse rifle to be next to useless, to name two. The Contact Beam is handy on occassion, but the charge time is so unwieldy that it really downgrades the usefulness. The Force Gun, too, while good for buying you time in battle situations, is a little underpowered.
Therefore, pick your three favorite weapons (I got the Line Gun, Plasma Cutter and it was a toss up between the Force Gun and the Contact Beam, but I tried both) and buy them and them only. The key benefit to this is that you'll only get ammo for these three guns whenever you find pickups. That means you'll usually have an abundance of ammunition for your weapons. Running out of bullets is disasterous, especially in Impossible mode, and can put a halt on your progress.
Constraining the number of weapons you have also allows you to sell whatever you have an abundance of, which is key. Which brings me to my second point.
2. Sell! Everything. I recommend keeping only health items and then only the biggest ones, maybe one small air can if you think you need it (I did -- one time in the ENTIRE GAME), and as much ammo as you feel comfortable with in your primary gun (i.e. whatever you can upgrade the highest). Other than that, keep maybe three reloads or less for each of your secondary weapons. The rest, sell. Sell everything. Get lots of money together and use it to buy suit upgrades, Power Nodes and health items if you're in a pinch.
3. Upgrade the most important things you have -- Stasis, your suit and your best gun. I recommend outfitting your Plasma Cutter with a high degree of damage and clip size and using it pretty extensively. Next in line should be your Line Gun with clip size and damage, as it can take out multiple targets. Otherwise, dump every node you can in to your Stasis and suit. You need to be able to take a lot of hits before dying, and more importantly, you need to be able to use Stasis to get you out of sticky situations whenever possible. Upgrade extensively and often. Don't spread your nodes around -- deal with what's most important to you first, then move down the line. Early in the game you should use the opportunity to get your suit strengthened up as much as you can. Ignore the Air nodes, as they're fairly useless. Buy Power Nodes whenever possible. The dividends upgrades pay are key to your survival.
4. Use Stasis like crazy. Sacrifice Stasis charge whenever you need to and whenever it will save you health. There are Stasis chargers all over the game, and more Stasis pickups, and staying healthy should always be your No. 1 priority. You'll want to use Stasis next to constantly, any time you're in a big battle, and any time you need a second to do some serious damage against multiple enemies. On this difficulty and with the number of shots it takes to bring an enemy down, the slowdown effect of Stasis is key to getting through the game.
5. Avoid unnecessary battles when possible and ignore Power Node doors unless you really need the stuff inside. There are a few points when enemies appear in areas you won't return to. For example, the second Brute in Chapter 4: Obliteration Imminent really isn't necessary to destroy. Your only reward for the massive loss of ammo and health is a single Power Node. Use Stasis to get past, Stasis again to complete the objective in the room, and Stasis a third time if you need it to get back out of the room. 
Similarly, there are plenty of doors that require power nodes to enter. These occasionally have items like Gold or Ruby Semiconductors inside, plus ammo, so sometimes it's worth the exchange of a Power Node. Sometimes, not so much. The plus -- maybe 2,000 credits in net earnings -- often is outweighed by the plus of plugging that Power Node into a gun or your HP counter where it can do you some good. The guide is helpful for that, but usually, unless you KNOW you'll get a Ruby conductor at least out of the room, you might be taking a loss. You need all the money you can get to keep your guns and health full and your upgrades coming.

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