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HardOCP Doom3 review it shows that you will most likely be able to step up to 1024x768 High Quality with a 5900xt (albeit they used an XP3200+ and 5900ultra with 256mb) from 800x600 Medium Quality of ti4600.
Far Cry - Nvidia - shows that 5900xt delivers playable framerates at 1024x768 when your card will not do more than 44 frames at the lowest resolution.
For the prices you've provided a 5900xt is the best choice no doubt. However, you shouldnt expect it to play games very fast after 1 years time from now. Then again you can always upgrade later. And I am not even mentioning the ridiculous prices of ati products. Also consider that 5900xt will give you increased framerates in all other games. The problem is that Far Cry and Doom 3 are very demanding (but doom 3 is forgiving at the same time). One other importance is the ability of 5900xt to support dx9.0 effects (like heat haze in doom3) and this will surely give you a better looking Half-life 2 and S.T.A.L.K.E.R gaming experience later on, as well as with those 2 games you mentioned.
EDIT: If you get $100 for your old card, spending $83 for 2x the performance in some instances and overall better gaming experience is definately worth it. Most people spent $200-300 when they upgraded from Ti series to 9700Pro/5900xt, etc. You can also think of it like this - even in US, a ti4600 costs about $83 right now, and in theory you are technically getting a 5800xt for the same value (assuming trade in).
I said prices are final cuz some people link up to newegG 
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