I want to build a wall in my basement to enclose my furnace and water heater. The furnace is gas powered for radiators. Just wondering how far the wall should be from the furnace and WH. Also, does it need to be built of any particular materials? Could I do wood studs with wood beadboard for example, or is that a fire hazard? Should I use a vented door? Do I need a vent bringing air in from outside? What is standard practice?

It would be best to check your local codes. You should use fire rated drywall though. Also, leave PLENTY of room around BOTH the furnace and hot water heater so someone can services it and/or remove it if needed. And plenty of room doesn't mean 8-10 inches. It means PLENTY of room, at least a foot and a half. One thing I HATE the most is when the customer decides that s/he doesn't like the look of their units and box them in tight to get as much room as possible, then they get angry when you have to knock a hole in their wall to get to something.

And yes, you might need a vent to supply air to the furnace and water heater or you will create a vacuum and dangerous conditions.

As a rule of thumb for clearance from furnaces and hot water heaters etc. The front should have at least the same distance as the entire debth of the device. The sides and front should be at least on half the width and debth for access. Over the top at least one inch of clearance is required for top of ducts and pipes. As Adam B said you need room to work. Even though it would be ideal for more space this would be the least I would do. Remember some of us contractors are a little wide in the middle, and the extra space makes it much easier to fix or replace equipment in the future.
As far as air supply. Ideally you should use a louver door for the access to the utility room. But if you desire not to go that route you need to determine the total input of BTU'S for all the equipment in the room. Once done for each 1,000 BTUS of input you need one square inch of fresh air. Both near the floor and near the ceiling. So for a 100,000 BTU input unit you would need a 10" x 10" fresh air grill both at the bottom and one at the top of the wall. Some townships requre that you use fire rated air dampers as a safety measure in case a fire breaks out in the room.
Also you should have as Adam B said fire rated sheetrock both on walls and ceilings with at least one coat of spackel on all joints.
On some equipment you can find the clearance requirements on the label or in the installations instructions. Be sure to check with the local code offical for their advice.
As some towns have their own requirements as well.

thanks for all the advice. maybe building this wall isn't the best idea. 30" in front of the furnace would take up too much space. i'm sort of semi-finishing my basement anyway - but i will be using it as an office.

perhaps i could suspend panels of something from the ceiling for a make-shift wall that would be easy to remove if work needed to be done? i would mind a curtain but that is a fire-hazard - but maybe i could use something similar that is fireproof. anyone have any suggestions for something like that?

Try using bifold doors without the track. Purchase enough to wrap around the the area. They will free stand and can be put fairly close to the equipment but no closer then 10 inches in the front of the heating system. They can be painted to match the area and can be taken down and moved if service is needed.