Overall, the 7800 GTX (Series 7) is mostly about increased efficiency and higher performance over the previous generations products. Normally we see little steps up in performance, yet with 24 pipes, 8 vertex units and a huge transistor count combined with some nice internal hardware optimizations, the boost in performance is noticeable, especially on the new 512MB version. I have no clue why it was not labeled "Ultra".

• 256-bit GeForce 7800 GTX 512 MB (550 MHz clock)
• 302 Million transistors
• 8 Vertex Shading Units
• 24 Pixel pipelines
• 512 MB 256-bit 1.2ns (8x32) GDDR3 Memory (850MHz clock - 1.7 GHz effective)
• PCI Express x16 Compatibility (PCI Express Compliant)
• Integrated NVIDIA TV Encoder (HDTV, S-Video, Composite,)
• Integrated NVIDIA VIVO (S-Video, Composite,)
• Dual DVI-I Connectors
• 54.4 GB per second memory bandwidth
• NVIDIA PureVideo Technology
• DVD + HDTV Decode assist up to 1920x1080p resolution
• Integrated Dual 400MHz RAMDACs
• 1 Dual Link TMDS DVI Connector
• 1 Single Link TMDS DVI Connector
• NVIDIA Digital Vibrance Control (DVC) 3.0
• Support for Microsoft Video Mixer Renderer (VMR)
• Advanced adaptive de-interlacing

What you likely want to know is what the technical differences are between the the "regular" 256 MB version and the "new" 512 MB version of the GTX. Basically, two things. The 256 MB GTX had it's graphics core clocked at 430 MHz and it's memory at 600 MHz (1200 effective). The new 512 MB GTX has a core clock of 550 MHZ and it's memory at 850 MHz (1700 effective). That is a HUGE bump in core performance and framebuffer bandwidth. That framebuffer bandwidth (framebuffer by the way is the local memory on the graphics card) was at an already fast 38.4 GB/sec and now is at 54.4 GB/sec.

An interesting fact is that the memory onboard is actually rated 900 MHz. So on purpose NVIDIA left you some overclocking abilities. This memory can run at 1800 MHz without any issues, guarranteed.