i turned the key got power to everything, radio came on and then click... i kept trying and no crank just a click... so i tried jumping the car and no luck

the dash lights would come on, but once i tried to start the car they would go off...

Nothing was left on except a dome light which my mechanic told me times-out after 16 minuts (think my GF left it on)... i was blaming her but it seems like its not her fault.

so i cant imagine my starter is dead or my alternator because i drove home at night last weekend and had no issues...

would a battery just go dead like that?

Called BMW roadside, and they wanted $350 to install a new battery...
I asked if they were serious and they said the battery is $212... i almost laughed on the phone... more for the fact that they wanted almost $150 to install it.

so my mechanic suggested hooking a jumper directly to the battery in the trunk, and then one to the front of the car and letting it sit for 20 min... then he said it should work.

I am looking for batteries on line and it doesnt seem like anyone has them for this car...

is it a big deal if i just get a regular 12V battery from sears or autozone?

Remove your battery and have it tested and if you have time take the alternator with you. Some parts stores can hook up your alternator and check the voltage regulator. For those that live in Colorado, Generator Exchange is awesome when it comes to rebuilding starters or alternators. They charge around $30 plus parts. They just did my alternator for $65 dollars and thats out the door, good people. :)

Dude what car is it and year

BMW batteries are built specifically for the vehicle and the differnt types of loads your car will strain it with.

It is always best to use factory parts, plus sears, auto zone, ect. sell some crappy batteries that you don't want in your BMW

Sounds like a dead bat and yes batteries often go like that. 5 years is pretty good for a factory battery. You can replace with an aftermarket, but then you've got to make sure it physically fits, the retainers hold the new bat, the venting system works and it's got the proper cranking amps that you need.

BMW batteries are built specifically for the vehicle and the differnt types of loads your car will strain it with.

It is always best to use factory parts, plus sears, auto zone, ect. sell some crappy batteries that you don't want in your BMW
I respectfully disagree with some of your sentiments...in that my experience with BMW batteries (over the past 20 years of BMW ownership) is that there ARE aftermarket batteries that are just as good as the original.

First, the oe BMW battery is mfg'd by Douglas Battery. The equivalent Douglas Battery 95R6 can be puchased from Batteries Plus and it usually runs between $80-$130..whereas BMW will extract approx $150-$250 from your wallet. The only diff between the two is BMW's battery case is white, and the Douglas case is black. :)

You are correct with inferring that "just-any-old-battery" won't do. But if you get the correct class size battery...you will do just fine. Johnson Controls makes about 9 or more of the leading battery brands...and if you purchase one in the right class size, you will have enough "cranking amps" to perform the duties required of the e38...plus not empty your wallet needlessly. For example Johnson Controls mfgs the:
1. Duralast 49 D sold by AutoZone (approx $69-$79)
2. Interstate MTP 93 (find a local reseller at www.interstatebattery.com) approx $80-$120
3. Advance Battery
4. DieHard
5. Optima

The Duralast 49D has been used by many e38 owners and is the best bang for the buck with its lowest cost...and it has 1000 cranking amps/850 cold cranking amps:
http://www.bimmerboard.com/members/feaqs/original/433594_219_full.jpg