From the 3.5 SRD
Hill Giant
Large Giant
Hit Dice: 12d8+48 (102 hp)
Initiative: -1 (Dex)
Speed: 30 ft. (hide armor);
base 40 ft.
AC: 20 (-1 size, -1 Dex, +9 natural,
+3 hide)
Attacks: Huge greatclub +16/+11
melee; or rock +8/+3 ranged
Damage: Huge greatclub 2d6+10;
or rock 2d6+7
Face/Reach: 5 ft. by 5 ft./10 ft. .
Special Attacks: Rock throwing
Special Qualities: Rock catching
Saves: Fort +12, Ref +3, Will +4
Abilities: Str 25, Dex 8, Con 19,
Int 6, Wis 10, Cha 7
Skills: Climb +9, Jump +9, Spot +4
Feats: Cleave, Power Attack,
Weapon Focus (greatclub)
Hmmm. Size "Large". That'll probably have to carry over or your player/NPC won't really feel like a giant. So make the guy size Large. The base giant abilities are pretty easy to go over: Str 25, Dex 8, Con 19,Int 6, Wis 10, Cha 7 means +14 STR, -2 DEX, +8 CON, -4 INT, +0 WIS, -2 CHA
For a half-giant I'd make that +8 STR, -2 DEX, +4 CON, -2 INT, -2 CHA
I'm not cutting everything exactly in half because I want to keep the cost of the template a little low. You make the template cost too high and you end up with a powerhouse with a -1 WILL save that gets taken out by the enchanter in the first round. That sucks.
I'd remove the rock tossing. Its a neat ability, but we're trying to convey that the half-giant isn't quite the size of a real one. So our half-giant is a small "Large" and a true giant would be a bigger "large". Kind of like how 4' and 7' tall count as size Medium.
Giants are all type -- wait for it -- "giant" which means that unless otherwise noted they all have 60' of darkvision. Chop that in half to 30', but we're going to leave the character as type "giant" because that's kind of cool.
So we're left with the following abilities
Type: Giant
Size: Large
Ability modifiers: +8 STR, -2 DEX, +4 CON, -2 INT, -2 CHA
Darkvision 30'
That's an aggregate of +4 on ability points. The DMG suggests that strength should be given more weight because its used all the time. I personally disagree with that assesment. Being size large is pretty nifty, your reach is huge and if the character uses a halbard or other reach weapon things will get very nasty, very quickly.
I'd put the template at +1 ECL if you're running a high-power game (32 point buy or more) or +2 ECL if you're running something more moderate/low (25 point buy).
Hope this helps!