tesla-geforce?which card? why? simple questions

I have some simple questions. As far as I understand tesla cards have some equal counterparts in Geforce family which are way cheaper than tesla’s. For example

9800 GTX and tesla c870 both have 128 Streaming processors.But tesla is nearly 1300 dollars where 9800 GTX is around 350$.
And the new card gtx280(around $650) has 240 cores which looks like equal to (still expected-this fall) tesla c1060.(price???cant even guess)

So what could be the reason to buy tesla instead of cheaper equal capability geforces?

I know one difference that tesla dont have direct video output. So does this mean it wont be limited by (I think it was 5 secs?!) running time? This leads to another question.

Is running time related to direct video output?Or is it only specific for tesla?(that would make sense that they are expensive)

if I dont use my geforce for direct video output(i can put another crappy video card for display) , can my kernels pass time-limit also??

Thanks.

Akif,

The specs are indeed similar, but not identical. The crucial difference is memory size: the Tesla has much much more, which can be invaluable for most kinds of computation.

For example, the GTX280 has 1GB, but the Tesla version has 4GB.