There's a huge price war going on at the moment with new cards launched by ATI (4850 and 4870) and Nvidia (9800gtx+ and 200 series). So it's a good time to buy.
The 8800 series cards are great but getting long in the tooth now, so I wouldn't get one unless your motherboard can't handle the newer PCI-E 2.0 cards. If you do go 8800 series, get a 8800GT (can be had for around ?100 now), the GTS and GTX are overpriced. I don't think your motherboard will have any problems with any of the PCI-E 2.0 cards as I looked the A8N-SLI up and it has PCI-E x16 slots and PCI-E 2.0 cards should all work fine in that slot (from what I've read). However, bear in mind that all these newer cards have high power requirements and some require two six pin PCI-E power leads, so make sure your PSU can handle this and has all the necessary connectors or adapters.
Best bang for the buck currently is between the Geforce 9800GTX+ (note the plus at the end, make sure you get this as they significantly improved the performance and reduced the price compared to the 9800GTX) and the ATI 4850 /4870. Personally I've always been loyal to Nvidia, their drivers are far more stable and their cards run cooler and usually pull less power. The new ATI 4850 and 4870 cards run VERY hot, so you'd need good case cooling if you go that route.
I recently built a new quad core system and put a Geforce GTX 260 in it, which is over the top for what I want to play now but I didn't want to upgrade again for a long time and the price wars meant the price on the 260 tumbled by over 70 quid in a matter of days and it dropped into my budget. The 9800GTX+ wasn't out when I built my system a few weeks ago, but I couldn't wait any longer. I don't regret the GTX 260, it is awesome, but I suspect the 9800GTX+ would have ample and might have opted for that myself.