Im pretty much decided on getting a 780i mobo with 4 gig of ocz sli pc2 6400 and a core2 E8400.
 
As you can see this isnt the latest technology as Im on a budget.  
 
Im unsure about which Graphics card/s to get.  
 
Im planning on building in a couple of months so prices/budget could change a bit, but at the minute I can only afford one card.  
 
My options are get an xfx OC 8800gt then get another when I get the pennies ( currently £140 each )
 
OR
 
get a 9800gtx and then get 2 more when I get the money( currently around £200 each )
 
Im thinking the first option will be most cost effective as when the 8800gt sli starts to struggle in games the next gen cards will be out. Also doesnt the performance gain per card in sli drop rapidly?
 
Any suggestions?

9800GTX > 8800GT, but for price per performance wise 8800GT is the best. Depending on your budget. I suggest single 8800GTS 512Mb, but the decision is yours to make.
 
For SLI thing, unless you SLI now, there no point to 'upgrade' second one when your first card start to struggle as at that time the newer and more performance card is already available and cheaper and probably will outperformed your present SLI configuration.

When I say I will get another later to SLI i meant as in when the next paycheck comes in not when the single card starts to struggle.
 
Im planning on building a new system in a couple of years and just using this as a temporary solution which is what made me lean towards the 8800gt.  
 
I was going to look at the GTS but its not too much cheaper than the 9800GTX and I got confused by some posts I read saying there were 2 different chipsets on the cards.

Tri SLI 9800 is rubbish... one single Radeon 4870 beats the cr@p out of even a 9800GX2 ... get that instead.  
 
Of course you can't Crossfire on the 780 board.

Oh and another thing, I would imagine a very expensive PSU is needed to run 9800GTX in tri sli?

Ok now Im seriously considering the GTS. When I said it was a simmilar price to the 9800gtx I was talking bollocks. Its actually £50 cheaper. But still £30 - 40 more expensive than the GT.  
 
Is this price difference worth it?
 
Also is there an easy way to tell if the card is using the new chipset?
 
Im going to look at GTS sli vs GT sli benchmarks

Just get 2 standard 8800gts, you can find them for £100 on www.ebuyer.com  There's no point spending the extra for an "overclocked" one, you can just overclock it yourself!
 
For £140 you could probably find an 8800GTS!

TRI sli 9800gtx is a total waste of money. To begin with the 9800gtx @ factory clocks is very underwhelming, buying a second gives you about 50-70% more performance for twice the cost. Getting a third bumps that up to only about 50-80% more performance than a single card for 3x the cost.  
 
The most sensible thing to do would be to buy an 8800gt and then either step up or trade up to the new generation of cards. If money is not a concern and you want the system now then go with either the 9800gx2 or 2 sli 9800gtx. Tri or quad sli is just not worth it at the moment.

8800GT, GTS and 9800 GTX are using the same G92 chipset, only different in core/memory/shader clock. The differences between three of them is not very much. IMHO and from what I personally suggested you earlier was 8800GTS because the stock cooler of 8800GTS is DS and you'll never have a heat issues compare to 8800GT stock cooler which is SS. If you happen to buy 8800GT stock cooler and replace the stock cooler with aftermarket cooler, the total price is almost similar to a 8800GTS while you'll get an extra performance over it.
 
For your SLI consideration, please read this http://www.tomshardware.com/forum/ [...] sfire-faqs
 
Finally, if you are planning to build your system in a Coming couples of years, why bother now to consider? In the next coming years, either 9800GX2 or 4870X2 will become obsolete.

Im planning on building soon to last a few years.  
 
The main reason Im not shelling out a load of money on the latest GPUs is I want to see how good the next gen is.
 
Im thinking Im going to get an xfx 8800gts then sli when I get the money.