Screw that great crossbow junk.
Go half-giant, from the Expanded Psionics Handbook (or the 3.5 SRD online).
Get a light crossbow of Large size, since you can use it fine as a Half-Giant.
Max out your Dexterity and Wisdom as much as you can, no other stats will matter much except Constitution. Just make sure Con is decent, it doesn't have to be great.
Class-wise, start out with 1 or 2 levels of Psychic Warrior, for the bonus feats, HP, and psionics. Then turn to the Ninja class from Complete Adventurer; the ability to turn invisible for a full round several times per day will help you make Sudden Strikes with your crossbow, boosting your damage output. Eventually you'll be able to poison some of your bolts quickly, which might help a bit too.
The Vigor power can help you survive at low levels without a great Constitution, and Offensive Prescience or Offensive Precognition will be your friend (I'd go with Offensive Prescience, for its damage boost, since you shouldn't have too much trouble actually hitting stuff).
Take Psionic Shot and Point Blank Shot as two of your early feats. Get Rapid Reload and Rapid Shot after those, that way you can fire multiple times each round. Then pursue the Crossbow Sniper feat from PHB 2. You'll need Weapon Focus first, though. Deadeye Shot and Telling Blow will also help. Improved Critical or the Keen magic weapon property will also help later, since crossbows have a decent threat range to begin with and Telling Blow pumps up your critical hits with sneak attack dice and such.
Get your Concentration skill as high as you can keep it, until you reach the point where you can easily make the skill check for attaining psionic focus. It'll allow you to use Psionic Shot frequently. Just remember to keep 1 power point in reserve at all times in order to attain psionic focus.
Later on you can get Psionic Meditation and Greater Psionic Shot or Deep Impact, to further boost your damage output sometimes.