Alright, for that thing you can save some money and get one of these;http://www.overclockers.co.uk/showproduct.php?prodid=GX-184-SP&groupid=701&catid=56&subcat=1274http://www.overclockers.co.uk/showproduct.php?prodid=GX-093-OK&groupid=701&catid=56&subcat=356http://www.overclockers.co.uk/showproduct.php?prodid=GX-187-SP&groupid=701&catid=56&subcat=938
The differences in price are fairly representative of their respective performances.
However, that monitor is pretty crappy. If you plan on upgrading it at any point in the not too distant future you may just want to go ahead and get that HD4850 1 gig. It's definitely massive overkill for that monitor but it is a good deal and these other cards probably won't cut it if you do get a decent monitor.
Another important question which might make all of this irrelevant what kind of power supply you have. There's a decent chance it may only be able to handle that first card I linked in this post.