I notice using a stronger soul gem while enchanting no longer makes the enchantment stronger, so what is the point now?

Higher maximum charge = more uses.

also the larger the soul gem, the larger the soul it can carry. and i heard tht black soul gems r required to capture human souls

Check again, There is a difference in enchanting when using greater soul gem, to common, to petty. Example is (not exact values) +8% mana regen when using petty soul gems, +12% when using common and +18% from greater soul gems. This is from lvl 15 enchanting( yes that is the starting level).

These numbers I'm about to use are completely random. Say a petty soul gem will enchant your weapon with 10 fire damage per swing, and you get 20 swings before your weapon needs recharging. Well a grand soul gem will still give you 10 fire damage per swing but give you 200 swings before a recharge is needed. The ratio is completely made up, I just added random numbers for a demonstration. I'm pretty sure that's how it works.

just get several types of souls in gems, go to the enchanter table and, switch the gems in the panel and watch the values change (that's for apparel effects, weapons work as Varus posted above)