Identification: Echinopsis... I think?

Hi,
I've had this cactus since it was about 1/2" in size and have misplaced and forgotten the name. I think I remember it being an Echinopsis but any help with full I.D. would be great.
:}

 

a pic of it in flower would be needed to make an id.

 

uh try again now sorry had to add pic...

 

Here is a close up of the cactus.
The flower is about 6 inches across.
Do I need to cross-pollinate these flowers to get them to set seed?
If so, will the plant cross with one of it's own offsets or any variety within it's species?

 

a fantastic flower. - Millet

 

The cactus looks like a Echinopsis subdenudata according to the shape and the flower. You have a nice looking plant. - Millet

 

Thanks,
I've got a couple of unknown cactii which I'll post photos of as they flower. I was working at a nursery that decided to buy over 1000 mixed cactii and succulents and have a few cactii I still haven't been able able to get definite i.d. on. Somehow it became my responsibilty to i.d. crash course learn prop and care of these guys. Having mainly grown sub-tropicals before then it was a definite change.
I did get some nice Pachypodium and Adenium out of the deal which was good.:}

 

Thanks to all for your help here's another photo. The cactus is there.... honest!