You definitely need to check out this thread:http://www.consolegold.com/forums/viewtopic.php?t=1739
I'm upgrading from a Geforce 4 Ti4200, and I chose an ATI 9800 Pro. I've got a P4 2.4 ghz system with a gig of PC2700 ram. The 9800 Pro seemed like the best DX9 capable card out there for the money that would match up with my system specs. I actually started out looking for a 9700 Pro, but they seem to be really tough to come by these days and are not priced competitively (often almost as much, or strangely, more than a 9800 Pro).
Another strong option is the new Nvdia Geforce 6600GT. It's a more powerful card than the 9800 Pro, but not overwhelmingly so (at least not by an extent I would consider overwhelming). My decision was made on a price basis in the end. I found a refurbished ATI 9800 Pro for $175, where the cheapest 6600GT I could locate was $230.
Also, you really should check out the Half-life 2 GPU shoot out at anandtech. Part 1 covers the most recent, bleeding edge cards (http://www.anandtech.com/video/showdoc.aspx?i=2278&p=1
), and Part 2 covers older cards like your ti4600 (http://www.anandtech.com/video/showdoc.aspx?i=2281
). There are also good articles linked in the other thread from Tomshardware.