Yep, gotta agree, that 3450 is a painful, painfully slow GPU for gaming in the first place, and with that new 22 monitor, it only gets worse! You should be able to get another 400mhz out of your X2 as well pretty easily, that will give you a nice little bump when you get that new GPU as well.
A Radeon 5750, for $130, will perform great on you monitor.
Most games at maximum settings
A Radeon 4770 or 4850 for $110
Most games at medium settings
A GeForce 9800 GT for $90.
Exceptional performance at 1680x1050
However, off all of these, I would recommend you to get a 5750.
Will be great for you for couple of years or so
The 5750 actually performs the same as a 4850. It's not really worth the price because you can get a 4870 512mb for around $120, and that performs equal to a 5770.
2.0 cards are backward compatible.
Unfortunately it seems that computer usually came with a 300w PSU which severly limits your choice of video card. You should be able to use an HD4670 however which is a decent card for lower resolutions like 1280x1024 and below. If your resolution is higher than that you will probably want to replace the PSU before upgrading your card.