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Essentially, it effectively increases bandwidth in the graphics chip enabling more complex effects to be applied more efficiently. The problem is that it's not possible to apply Multi-Sampled Anti-aliasing to effects done this way. It theoretically is possible using Supersampling, but the performance overhead is so great that the driver won't let you use it. DX10.1 solves this problem by enabling multiple fetches from the G-buffer. The benefit to the user is deferred rendering with MSAA, so we can get great effects and improved image quality at the same time.
Additionally, in DX10.1, 4x Multi-Sampled Anti-Aliasing is now required on all compliant hardware. This is great as decent performing MSAA will be made available at a lower base line level.
To get these features you need specific new hardware, which gives AMD something to bash nVidia over the head since despite it's dominance in performance, it has yet to add DX10.1 support to any of its cards.
A similar situation occurred when nVidia had Shader Model 3 support on its 6800 and 6600 card, while ATI only had Shader Model 2 on X800. Now there are games out there that require Shader Model 3, but that's unlikely to be the case for DX10.1 for a while, as developers have been quite vocal in stating that it isn't seen as something crucial for them, which is a shame as it does bring some cool things to the table.
However, by the time games that take advantage of DX10.1 hit the market this card is likely to be quite dated so I wouldn't recommend basing any purchasing decision on this feature. If it is an issue then perhaps you should hold off as rumour has it, and let me stress that this is rumour, that nVidia's next gen card will appear in a couple of months and will offer DX10.1.
With the new architecture AMD has also promised that the card will support up to four cards in a format called CrossFire X. This will be limited to working just with the Spider platform, so you'll need a 790FX chipset and AMD CPU to get it to work. Driver support is promised in January so we can look forward to seeing it in action then.
On every card you'll find two dual-link DVI connectors, so you can hook up to two 30in monitors and HDCP support is built-in to every card so there won't be any problems watching your high def discs, especially with UVD on board.