"nforce" is just a brand name Nvidia uses to name it's motherboard chipsets.
To simplify heavily, motherboards are made roughly as follows:
A company (such as Nvidia, VIA, SIS or Intel) designs a chipset, and usually makes the key chips such as Northbridge and Southbridge chips. Other companies (such as ASUS, Abit, Gigabyte, MSI and so on) then design the actual motherboards, making use of the chipset designs and the chips they buy from the chipset designers (aswell as other components from other suppliers).
In this way you have manufacturers like Asus producing boards based on many different chipsets. A motherboard that has an nforce chip on it is not necessarily "better", because those that dont will have somebody else's chip/design instead.
To use the two motherboards you give as example, the Asus K8V SE Deluxe is a motherboard made by Asus using a chipset called K8T800 which was designed by a company called VIA. The other board, the Asus K8N-E Deluxe, is based on the Nforce3-250 chipset which was designed by the company Nvidia. You will also find other manufacturers have motherboards based on chipsets by VIA and Nvidia.
Which is better? I dont know, though my guess would be it depends on what you need and what you want from the motherboard. MY suggestion would be to examine the features of each and eliminate the motherboard options that do not have what you need, then check some websites for performance indicators to see which of the suitable boards are faster, and also check some user comments on a board like this - hopefully some of which will follow this post
Hopefully that explains the situation to you and helps you make a more knowledgeable decision