Tyres probably make the most difference of any modification you can do to your vehicle. Good tyres make a huge difference. I've seen tricked out vehicles with twin diff-locks but poor tyres fail to get through an obstacle and a normal vehicle with good tyres go straight through. 

Taller tyres get the diffs further off the ground. You don't bottom out so much. They do raise your gearing, and lose some turning circle and braking - both less engine braking and greater rotating mass. I adjusted the lock stop bolts to prevent the taller tyres rubbing on the radius arms and fitted lower diff ratios (4.11's) to compensate for the gearing. Upgraded brakes and caster corrected steering arms were also required.

Fitting tyres that are too big is a common problem for new 4WDers who haven't been given good advice - not only do they touch the body but they also changing the driveline geometry and may overload drivertain components. Before you start bolting bigger tyres make sure you understand the issues - like the legal issues (recertification) and insurance impacts, the impact on geometry. I'd talk to experienced 4Wders either at your club or online first!

After starting out with 205/16's (why do Landrover even bother with those?) I tried 225/75/16's. Not much better. Asking some discussion I found that 255/85/16's BFG Mud-Terrain's fitted perfectly on an unmodified 90. They *just* rub on the rear flares and also the top underside of the back wheel arch back at full articulation. Sort of an audible wheel travel indicator. They fit on standard rims too! They work out to be 33" (the 205's are 29" by comparison) That's two inches free clearance under the diffs. They are narrow enough to really dig-in in the mud as well. They are even reasonable on sand. Dropping them to 14 psi makes a big difference.

However, ongoing modifications (aka the arms race) being what they are, the quest for bigger tyres keeps marching on.

I splashed out on some Simex Extreme Trekkers in 35 x 10.5 , but they required some modifications to the suspension and drivetrain to fit well. Awesome in the mud though. A set of SecondAir beadlockers helps - low pressures & simexes are made for each other...

I decided two sets of tyres was enough and sold the 265/75 All-Terrains. They are a great tyre and I would seriously consider another set if I could find a 34/35 x 11 or so size.

I upgraded to a slightly wider 285/75/16 BFG Mud Terrain for general use when the 255's got worn. Very happy with them so far, run them without tubes on standard rims.

For the Disco I stayed with the BFG MTs but in a smaller size - 265/75's. Overall I am very happy with the BFGs and have had a great run with them. Recommended.