What's the big difference here?

Hi i have a pretty lame 256mb graphic card in my computer, so i would like to switch it. My motherboard is a MSI P35 NEO. I wanna buy a card with enough performance (atleast 1gig memory) but it cant be too expensive either. And i've ended up with 2 very priceworthy graphic cards. One is the

Gainward GeForce 9800GT 1GB PhysX CUDA, it's detalied information is (sorry for bad english):

Clock Speed : 600 MHz
RAMDAC Clock Rate: 400MHz
Installed video memory (maximum): 1 GB - GDDR3 SDRAM
Strandards: Plug and Play, CE, FCC, RoHS

Price is about 140USD wich i think is a very good price for a fine graphic card

And there is the Gainward GeForce 9400GT 1GB PhysX CUDA (notice 9400!)

Clockspeed: 550 MHX
RAMDAC Clock rate: 400MHz
Installed video memory (maximum): 1 GB - DDR2 SDRAM
Strandards: Plug and Play, CE, FCC, RoHS

Price for this is much lower, it's about 61USD

So i'm wondering! Why this big price difference???

I'm not in to this so much but what i've discovered the 9400GT has 50MHz less Clock Speed, and only have the DDR2 SDRAM instead of the GDDR3 SDRAM that comes with the 9800GT. I know 9800GT is also a newer card then the 9800 but i wonder what is the big difference between GDDR3 SDRAM and DDR2 SDRAM?

I guess that's why the price is so different because i've seen many videocards that costs up to 200USD and still have 550MHz instead of 600MHz (Clockspeed)

Hope you understand what i mean, Clockspeed might not be the right term but in swedish it's Klock Hastighet, if that's helps some one. (stating the obvious)