Hi sometimesuk, welcome to itsallPC.

The main factor in graphics cards is their chipset (Radeon 3800, GeForce 8800 etc). Memory is only of secondary importance. For each chipset there is an optimal memory size - cheaper models have less memory, extravagant models have an unnecessary amount.

RTS are generally not very taxing for 3D graphics so pretty much any card on the market would do fine. Far Cry is still fairly taxing with all the settings at their highest and HDR turned on.

By far the best AGP graphics cards on the market today are the AMD Radeon 3600 and 3800-series cards; to the best of my knowledge there are no official AGP variants of Nvidia's current generation. A Radeon 3860 would be within your budget, and would perform reasonably well. A Radeon 3850 would be a little outside your budget, but is significantly faster than the 3860.

In your price bracket, there would be no reasonable reason to have more than 512 MB; 256 MB would probably do the trick.