The 9800GT is just a re-branded (renamed) 8800GT. They were nice cards, but they're dated. It's going to struggle with modern games; however, if you want to go that route, for roughly the same price (assuming $100.00) you can get a GTX 260. The drawback is that they're both DX10 and not DX11. Relatively speaking, they also both a bit louder than what you're currently using... not that they're loud cards, it's just that any card with a fan is going to be louder than an onboard graphics chip without a fan.
Newegg.com - PNY VCGGTX260CXXB GeForce GTX 260 896MB 448-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
Newegg.com - GIGABYTE GV-N26UD-896M GeForce GTX 260 896MB 448-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
If you're into all the sweet DX11 visual effects, then the 6670's are in your range. They may not be able to run all games at the same fps as the 9800 or the 260, but games will look a lot better:
Newegg.com - ASUS EAH6670/DIS/1GD5 Radeon HD 6670 1GB 128-bit GDDR5 PCI Express 2.1 x16 HDCP Ready Video Card
Newegg.com - XFX HD-667X-ZHF3 Radeon HD 6670 1GB 128-bit DDR3 PCI Express 2.1 x16 HDCP Ready Video Card
Imho, the most attractive card in the 100.00 range is the GTS 450:
Newegg.com - EVGA 01G-P3-1351-KR GeForce GTS 450 (Fermi) 1GB 128-bit GDDR5 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
nVidia has a driver setting called Ambient Occlusion which is disabled by default... you must enable and select a setting, performance or quality. Many games have ambient occlusions and enabling them has a really dramatic effect on lighting... it's
really nice.
ATI cards do not have this feature... and a bit off topic... Modern Warfare players are all up in arms about this setting because of its effect on smoke in game. ATI guys just see a thick white wall whereas an nVidia user with ambient occlusions enabled sees more lifelike whisps of smoke. At times smoke can be seen into, and this is at the heart of the argument... it provides an unfair advantage.
These picks were all done based on the assumption that your new monitor will be 1680x1050. If you get one that's 1920 or greater, then you're going to have to bust open that piggy bank and increase the $100.00 figure.
Anyway, here's a chart of gpu performance. Take it with a grain of salt, this is only a synthetic benchmark and so real world stuff may differ vastly. Before you buy anything, read several reviews and, more importantly, customer feedback. If a ton of guys are sayin the card sucks, you don't want any part of it.
http://www.videocardbenchmark.net/video_lookup.php?gpu=Radeon+HD+6670