Nvidia makes the chip only, so you won't see Nvidia branded vid cards. Only "3rd party" ones.
ATI on the other hand started out making their own card only, but in order to compete with Nvidia, they began selling the chip to other manufacturers as well. ATI video cards used to be very bad (driver and quality) but starting from the original radeon series, they're pretty much the best you can buy for the money in terms of quality.
Since you only have a 2ghz p4, you probably don't need the fastest card out there since the cpu will become the bottleneck anyway. You may as well spend $200 on the card and the remaining $100 on another 512M RAM
The 9800SE cards are slower than the 9800Pro's. It's about same performance as the 9600Pro