GDDR3 vs GDDR5 and 512mb vs 1gb

I am building a system currently, and have been wondering what the benefits of these are.

gddr3 and gddr5 - it seems to me that I have no real clue what the difference is

getting a 1gb card vs a 512mb card - A bunch of deals at the egg right now for 4850s with 512mb for ~115

Id like to get a 4850 or 4870, and assume getting a 1gb version would be better. I run a lot of CS3, Painter X, Maya, and lightwave. And I do love me some good video games. From my understanding, a 1gb card is better.

I want to buy a single card right now, with plans to add a second for CF setup.

My current setup that i am building is:
MSI P45 Diamond MOBO
PC Power and Cooling SILENCER 750W QUAD S75CF
Intel E8400
8gb Corsair DDR3 (4x2gb)
Antec 900 case
2 Lite-on Burners
1 WD 500gb HD
1 Seagate 1Tb HD

Not sure if i will use xp 64bit (most likely) or Vista 64bit
I have each of them from SChool!!(Says woohoo, Free OS's)

What do you guys and gals think??? I am so tempted to buy a 4850 512mb, but would also love to get a 4870 1gb for the same price as two 4850's and wait a year and add another 4870 1gb

If you`re going to install 8Gb of memory you will need to use a 64-bit OS, a 32 bit OS can only `see` a total of 4Gb, so the extra 4Gb will simply not be recognised or used.
GDDR5 is faster than GDDR3 and is the main reason the HD4870 is faster than the HD4850, otherwise the two cards are much the same.
If your monitor is less than 16x10 there`s no real reason to go for a 1 Gb card, the extra memory is really only used at higher resoloutions.

Good point on the 64-bit operating system. However, the HD 4870 is faster than the 4850 not because of its memory, but because of the increased clock speeds (750MHz core vs. 625MHz core). The GDDR5 does nothing (or very little) performance-wise, which I believe is due to its increased latency over GDDR3.

Ahh yes, I forgot to include that in my last post. +1

I was thinking of using a quad core processor, but most programs do not really use the 4 cores yet. The e8400 i already have and can be OC'd really well. The whole system will be watercooled, which should help for good OCing.

I want to run dual 24's at 19xx x 1200, and gaming isnt the most important (although it is in the back of my head ).

so from what i am reading, a 4870 with 512 mb is going to be better than a 4850 with 1gb?

I do want to eventually CF the cards, and the card hierarchy chart says that CF'd 4850s are a little better than a single 4870.

Then there is the whole issue of the 4830 CF'd, which after reading here and other places that that card shines in the OC'ing department