Manual says:
• Perform 25 and bard Level 16: +1 Will Saves, +4 temporary
Hit Points. +1 Dodge Armor Class.
• For each additional 5 Perform and 1 class level in bard an
additional +2 temporary Hit Points is granted.

Is this mean that a level 16 singing Bard with 50 points on Perform, is only as good as a level 16 singing Bard with 30 Perform points??

Thx

Quite simply, yes.
However, you can only improve your perform skill by 1 point per level (except when you can take feats such as Artist, Skill Focus: Perform and Epic Skill Focus: Perform, or invest attribute points in charisma). That makes it very unlikely you will get those +5 skillpoints per level, and so a bard with 50 points of perform will end up with a better bard song than the bard starting out with 30, once a few more levels are gained. In other words, there's no advantage at level 16, but a level 21 bard with 35 skillpoints does have a worse bardsong than a level 21 bard with 55 skillpoints.
The difference is not that great though, at +8 temporary hitpoints per ally. It becomes more significant when using the Curse Song, where it lets you deal an additional 8 damage per song to all enemies in the area.

since i'm now playing the official campaign with a multiclass bard */fighter 1/rogue 2, i guess it's best for me to invest skill points on Perform, only up to 30 points (including any modifiers).

am i right?? or is it better if i put some more points??

That totally depends on what level bard you're planning to reach. If it's 16, perform 25 (including charisma modifiers and feat bonusses) is enough.

You do realsie you are getting a 20% XP penalty with your character ?

whoa.. all the replies on my "triplet multiclassing" topic on this forum told me that my Bard */Rogue 1/Fighter 2 character doesn't suffer from XP penalties, since i use a half-elf.

now it doesn't matter, though, for i have earn 190000+ exp. points, although my Bard still at her 5th level.

I stocked up the XP to save my skill points and in the end, use them as efficient as possible.

that's why i start this topic.

anyway, thanks a lot for the great info. i'm thinking to put my skill points on Perform only up to 30 (best for a lvl 17 Bard).
the rests will mainly go to concentration, discipline, persuade, & use magic device.
few will go to heal and lore.

i think it will be the best for me. any suggestions or critics??