Letting the X800 Pro represent ATI is very cheeky indeed...

• ATI Radeon X850 XT Platinum Edition
  The final evolution of ATI's Pixel Shader 2.0 hardware, the X800 XT PE is powered by the lightweight 160 million transistor R481 core, which churns out 8.4 Gigapixels of raw power at 540 MHz. Comparatively, the 7800 GS can push 3 Gigapixels and the 6800 Ultra can push 6.4 Gigapixels, weighing over 300 million and 220 million transistors respectively. The Achilles heel of the R481 is the now dated Pixel Shader 2.0 architecture, which gives it a weakness in new and emerging titles. However, with a high fillrate, low power usage and low heat production, the X850 XT PE is definitely best-in-class.

The Radeon X800 Pro was a real dissapointment from the start, and has been since put out to pasture. A technically superior and fast solution in the Radeon X800 XL has replaced this SKU completely in ATI's lineup.

• ATI Radeon X800 XL (X800 XL 512)
  Never be tempted by a Radeon X800 Pro if a Radeon X800 XL is in the question. You may be thrown by the fact that the Radeon X800 XL is clocked at 400 MHz, while the Radeon X800 Pro is clocked at 475, however, the former has all 16 Pixel Pipelines enabled, while the latter only has 12. The heart of the X800 XL is the R430 Core, a special version of the R423 used in the PCI-Express X800 XT Platinum Edition that is built on a 110nm process (later used for the X1800-series). The standard model is equipped with 256MB of GDDR3, but is aptionally available with 512MB.

The only 512MB 7800 GS I have seen so far is the Gainward model, which is in truth a 7800 GT. There's a newer model coming out they are calling the GS+, which is actually a 7900 GT, which has 24 Pixel Pipelines at 450 MHz and exceeds the 7800 GTX in performance.

Incidentally, if by upgrading to 2.5 GB you mean you're expecting a pair of GB modules, and you plan to use them with your existing module, that would not be a good idea. This would mean you have an uneven number of slots populated, disabling the dual channel support of your i865 chipset. This would halve your peak memory bandwidth.

It should be noted that graphics glitches noticeable in NVIDIA cards are the fault of the drivers, and not the card itself. The shadow problem that Schmidget mentions reminds me of a problem that can occur in some games that handle Stencil - a feature for generating shadows by casting vertex buffers - incorrectly. Try enabling Stencil in Unreal Tournament 2004 and you'll see a similar thing with an ATI Radeon X800-series card. If you want to blame someone, blame the game developers for failing to implement sufficient checks or do sufficient testing. It's a shame that us, the consumers, are forced to choose between two excellent pieces of hardware on the whims of developers.

edit - wow. excuse my initially lazy typing. Should make sense now :P