you can't "add" internal storage memory... internal memory is used for some things that you just can't (safely) avoid...
* PIE (Internet explorer) cache - clear the cache frequently if you do a lot of web serfing.
* Message storage (attachments can go on your card) - keep just a few days worth of email
* PIM data (contacts, calender, todo) - nothing you can do here, but usually this is at most a meg or so
* applications that run in the background, or on the Today screen - installing these on the storage card can cause lockups
* ringtones, notification sounds - also can cause problems if on the card, keep 'em small
Given all that, you'll want a 2gig card (these are under $40) where you can install all your *other* apps (other then the today apps), your mp3s, your photos, your notes/docs, your email attachments, audio books, etc.
If you're of a technical bent, you can "cook your own ROM" and move a lot of the apps you use regularly into the operating system ROM, where they take up no storage space at all. I did that myself, and my custom rom with the apps I use all in it leaves me with 33+MB free internal store, which doesn't hurt speed... apps load from ROM a bit faster then from the storage card, that's for sure
In general tho, if you're not running out of space, your ppc will not run much faster or slower with a lot or a little free internal store. 6MB isn't a ton of space, so you should check for stuff to delete or move to a card (I know, for example, the Sprint version has about 6MB of "Audible" sample files that can be moved to card or deleted...)