I thought it was a bit odd that you'd take the time to write this as I've already posted such a thing several times in the forums. This website has answers to nearly all the SLI questions you could come up with. http://www.slizone.com/page/slizone_faq.html
Frankly, I would have just told people to go there instead of trying to write what you wrote. As GGA said, there are some problems, so lets get at them.
BUT the 7800GTX 512 will lower its clock to operate with the 256 one , so it wont have its true power.
Incorrect. The 512MB card won't lower any of its clocks at all. It will simply use only half of its ram. The clocks will stay the same, so it will still use its "true power". I would also mention that to do this, you have to use coolbits. If this were to be a sticky, I would post what you need to do in coolbits to make this work. From what I've heard, GF8 cards don't work with coolbits, so I'm not sure this can be done. You also failed to mention whether this is possible with Crossfire.
just make sure they have the same memory and same clocks
Uhmmmm, I thought we said memory and clock speeds don't matter? I know what you are trying to say. Why spend the money on a card with more Vram if you aren't going to use the extra ram? If you already have a card with 256MBs of ram, buying another card with 256MBs of ram would be more cost efficient, as buying one with 512MB would cost more.
WARNING: Some RAMs have SLI or CrossFire logo on them.
Seeing as you brought these up, you need to explain (briefly) what these are.
WARNING: Therer are some Power Supplies which aren't in the List
There are, or There're
(I dont mean SLI or CrossFire wont be good for resolutions like 1600x1200 or lower , i am just saying that SLI or CrossFire shines in higher resolutions .
You have a starting bracket, but no ending one. (you are missing one of these ")" ) At the start of the next sentence you start with the word Also. This is a grammatical no no.
Do SLI or CrossFire double the memory ?
This may not be the case for all X1950 varieties out there though.
There are three x1xxx cards that support crossfire without a crossfire edition card. The first is the x1950pro, next is the x1950GT, and the last is the x1650XT. These are the last three cards to come out from ATI, and the only three that have the internal crossfire bridge. The only card(s) that allows for Crossfire is another card with the same chipset. Read this for more info. http://ati.amd.com/technology/crossfire/howitworks.html
(look for the squares that are colored red in the middle with a dot in them.)
Seeing as at one point you used the wrong abbreviation for crossfire, I thought I'd point out the right one. The first few times I'd write Crossfire, but after that, you can switch to CF. I'd also try a bit more with the "2 vs 1" section. Getting two 8800GT is cheaper then a single ultra, and is probably a much better idea. Last, you seem to be writing this a a SLI page, if its going to include CF information, include CF information. Reading this, it seems to be SLI first, with a CF afterthought.