Yeah, it sure can. The PCI-Express 2.0 x16 slot on there will take any PCI-E-based graphics card. The fact it has an ATI GPU built in won't matter as that will be disabled (manually in the BIOS or automatically) when a graphics card is installed. You will, of course, have to install the latest Nvidia drivers. I recommend getting them directly from Nvidia since they are updated very often. I currently have a Gigabyte GA-EP45-UD3R with an MSI N275GTX TwinFrozr OC GeForce GTX 275 ( http://www.newegg.com/Product/Product.aspx?Item=N82E16814127429 ). It's noticeably faster than the Nvidia 8800 GT it replaced. The cooler that MSI put on there is much better than average meaning you won't have to buy a 3rd party card cooler (like I did for the 8800 GT, which ran way too hot).
That said, if I was buying a card in the $270 price range today, I'd take a very serious look at (and likely buy) the ATI HD 5850 cards. Here's what Newegg has: http://www.newegg.com/Product/ProductList.aspx?Submit=Property&Subcategory=48&Description=&Type=&N=2010380048&srchInDesc=&MinPrice=&MaxPrice=333&OEMMark=1&OEMMark=0&PropertyCodeValue=679%3A49753 Notice that most of them are out of stock. That's because according to these articles:
http://www.anandtech.com/video/showdoc.aspx?i=3650&p=1
http://www.tomshardware.com/reviews/radeon-hd-5850,2433.html
http://pcper.com/article.php?aid=788
the HD 5850 keeps up with or outperforms the Nvidia GTX 285 (which is faster than the 275) for the same price range as a GTX 275.
If you are interested also check these stores for stock:
http://www.mwave.com/mwave/ViewProductGrid.asp?DPDID=vid&Criteria3=ATI%20Radeon%20HD%205800%20Series
http://www.ncixus.com/search/?categoryid=2-108&q=5850
http://www.tigerdirect.com/applications/SearchTools/search.asp?keywords=HD+5850&searchbtn.x=0&searchbtn.y=0
http://search.zipzoomfly.com/?Key=HD+5850&Category=00&x=0&y=0
These cards just came out and the word is out about their price/performance ratio, so expect them to be a bit hard to find.
If you still want the Nvidia GTX 275, the introduction of the ATI HD 5850 almost certainly means the GTX 275 prices will go down. I still prefer Nvidia over ATI just because I tend to have less problems with the drivers. The prices are just likely to drop $40-50 on the GTX 275s in the next couple weeks. Myself, I'd pick up the 5850. It's shorter than the GTX 275s tend to be (i.e., it will fit better in the case and have better airflow) and uses less power (which generates less heat). I suggest sticking with the top manufacturers: Asus, ATI, BFG, EVGA, Gigabyte, MSI, XFX and I'll add SAPPHIRE as their reputation has been improving steadily.