Depends on the phone you currently have. Do you really need everything that you have downloaded?

The biggest difference you will find is the ability to install apps to your sd card.

You can partition upto 1gb of your sd card to be used by your phone as an extension to your phones internal memory.

Although some apps don't work properly and still need to be installed on the phone memory, like swype.

In addition, with a custom rom, you can move parts of the android operating system to the sd card also, like some cache files.

On a samsung galaxy portal/spica with something like 180 or 200mb of internal memory, aster rooting and custom rom, (2.2) before I install anything else, I have 143mb to play with. after I install what I need, I still have 120mb ish as most of the apps use the sd card partition, where as, unrooted, I could be down to 5mb free.

Hope this gives you some idea.

 

In answer to the original question, rooting in itself will give you no more free space.

Why am I saying this? Well I keep seeing posts saying "rooted but no more space". I think its best to clarify this.

To get more space, you must look at custom Roms with a2sd+ or s-off and flash a custom alpharev hboot table.

 

depends how far you want to go.
if you want to used a2sd+ you will need to create and ext3 partition and flash a rom with a2sd+ support
this will move part of the apps to a hidden partition on the sdcard (not like froyo apps2sd) it moves more data and will give you a little more head room to install apps (it will look like its installed internally).

Some roms support darktremor a2sd which will move the dalvic to the ext partition which will save even more space (if they dont use darktremor they may have a zip to move dalvic)

if you s-off using alpharev you can make custom mtd partitions which resize System, Data, Cache. this means you can flash a smaller rom and change the partition sizes to suit. this will give you tons more room for a2sd to work and youll have more space than youll use
or you can do like me and do away with a2sd and just create a massive data partition (but rom needs to be small to get enough room)

theres a few options

some rom

 

+1

 

Have a read of the ll the threads linked in the root sticky. Lots of valuable information there. Especially in the general section