Well my only thought is that a Geforce 4 TI 4200 will indeed show marked improvement in games over a 3 ti 200. However thatís not saying I recommend buying a TI 4200 at this time.
I think you can find an ATI 9600 non pro or pro for a decent price, also I have seen 5600 Ultras (Including the one in my rig) selling for between 118.00 to 130.00 dollars in retail package.
A bump up would be the ATI 9600 XT or Geforce 5700 and 5700 Ultra. But by the time you get to the 5700 Ultra you can find a 5900 NU for almost the same.
I really think you should be looking at ATI 9600 or 9600 Pro based on a moderate to low budget card that also has good performance. Kick up your expectations a notch and you open up a new can of worms.
Game - BIOSTAR TA785 A2+ / Phenom II X4 965 Black Edition Deneb 3.4GHz 125W / Patroit 8GB DDR2 800 CAS 4 Timing: 4-4-4-12 / Sapphire HD 6870 1GB / Antec Three Hundred Case / PC Power & Cooling S61EPS 610W / X-Fi XtremeGamer / Western Digital 640GB 7200 RPM SATA 3.0Gb/s /Windows Vista home premium 64 SP1
Back up - Biostar 6100-939 / A64 3800 Venice(Stock 2.4) / 1.5 GB Corsair XMS 3200 / HD3870