Hi fas,
Ghost Recon Advanced Warfighter only needs a Shader Model 2.0 card with at least 128mb of video memory. The Ti4600 series is simply a Shader Model 1.1 card, so this is the only problem you have.
As your system has an AGP slot, how far you upgrade for a new 3d videocard will only be limited by your budget, and as previously mentioned, how strong of a power supply you have. Beefier 3d cards require a lot more juice!
I'd also strongly urge for a 256mb vram card as this game has some extreme textures that can tax a 128mb vram card a little with some higher game settings.
Luckily, if your budget is low and you're unsure of your power supply, you can find a 256mb 9600 Pro or XT for cheap and these have Shader Model 2.0 support and draw just about the same power as the Ti4600 series. It will play this game quite well with medium to high settings, but wont be very forward thinking.. but it'll work.
If you have a stronger power supply, you may wish to consider an X800 GTO, 7600GS or as previously mentioned, the 7800GS is the most powerful AGP card you can get (and also goes all the way up to Shader Model 3.0, with support for 2.0, 1.4 and 1.1 also)... but they are expensive (usually $280 or more) and need a lot of power.
Hitting Newegg.com if you're a US poster will be your best bet. Look for AGP 4x/8x cards within budget and also inspect your power supply for how many WATTS it has (this will usually be like 200W, 250W, 300, 350 Watt etc.etc.). It's usually on a sticker on the side of the power supply so opening the case may be needed.
Good luck and enjoy the game! It's a lot of fun.