hello ive seen some other posts on websites but none really helpful or all the same looking for the answer to why my 3870x2 1 gig only shows up as 512 memory in the catylist control panel???

That is 512  per GPU, since there are 2 GPUs on the card the total memory is 1024.  
A bit of PR/marketing trickery IMO. The 512  from GPU #1 is used only by GPU #1 same for the memory on GPU #2  
Memory in a multi card crossfire setup does not compound. The same applies to what you have, Crossfire on a single card.

Normal.
 
It's not really 1 gig, but rather 512m x2.
2 3870 cards in crossfire sharing one board.
 
2 gpu's 2 512meg frame buffers, and even 2 BIOS.
 
the "1 gig" is more marketing than anything, but it is of no consequence really.

On my catalyst it shows 512 mb plus the core clock at 300 for 3870x2.

thats the idle speed of the core, load up a game and it jumps up

I have just bought an Asus Radeon 3870 X2 1gb and am having problems running any 3D Mark benchmarks. Every time I disable the Catalyst A.I setting my computer would run it fine however I am getting a low score of 12990 when my computer performed better on the same system with a Geforce 8800 GTX and got in the 14000. However when I enable the Catalyst A.I to either standard or advanced I get higher frame rates but everytime it reaches the second test ( the firefly test ) my whole computer would just crash and restart. Why does this happen? and also Im hearing small buzzing noises around my graphics card. dont know if it is my card or something else on the computer but Ive never heard it before until I installed this video card.
I am running:
Intel Core 2 Duo E6850 @ 3.5ghz
Gigabyte X38 DQ6 Motherboard
4gb G.skill ram
2x 500gb Seagate hdd in Raid 0
Asus Radeon 3870 X2 1gb
Creative X-FI Platinum Sound Card
Gigabyte 800 watt PSU

What drivers and O.S. are you using?  Your CPU is overclocked; are you sure you are providing sufficient voltage?

im using windows vista ultimate 64bit edition. i have both the 8 & 6 pin power connectors on the card and i do have my cpu overclocked from 3ghz to 3.5ghz...had it on 3.8 before but moved it down cause it ran hot. an 800watt PSU should be enough to run an overclocked E6850, Radeon 3870X2 and 2 hard drives?

Do you have a way of measuring the GPU temp?  Maybe your fan isn't working correctly and overheating.  I've never noticed a "rattling" from the card.  Do you hear the fan spin up during the tests?  800 watts is enough, assuming it's working correctly and is "good" power.
 
You should test the situation with your system set back to default-non-o.c settings and try to duplicate the problem.

Hushbrown, use rivatuner (google rivatuner + 3870x2 to get it to work) to set the fan to atleast 35%, it will be alot louder, but your card will rund much cooler.

 
So, if I have a 3870x2, it means the computer or game only uses 512mb of GPU ram?  
If that's the case, isn't it a waste of purchasing any Video card that has two cores?
 
Could somebody please confirm this? And elaborate further, as I'm a total noob

running the 3870x2 is like running 2 video cards
 
 
If you had two 3870HD video cards, they would also only show up as having 512mb of memory - in lamens terms, this is just how it works when you use more than 1 video card, it doesn't get more ram.
 
2 video cores lets the other functions of the video card stack together - somtimes it is cost effective to add a second card over going to the next tier of graphics for the best performance if you already own a specific card.
 
A lot of this would take more writing to explain than I care to be bothered with and there is TONS and TONS of documentation out there which explains this in great detail already. Long story short, you need to do your own research and be a bit tech savvy or you're not going to be able to make an educated decision when you do buy graphics upgrades.
 
 
On a plus side, the 3870x2 is a pretty good video card in all truth; so you really didn't go wrong at least.