Few computers can play Crysis very well to begin with. You have a couple of issues that could be causing a slowdown. First, your CPU, as you mentioned, isn't that good. It's got a slow clock speed and isn't using the latest architecture. I would go for the AMD Phenom II X4 955 Black Edition Deneb 3.2GHz
CPU if you are using demanding, high performance games. Crysis is one of the few games that can use 64-bit technology, therefore, to squeeze every last bit of power out of your system, you should be using a 64-bit version of the operating system and 4GB or more of RAM. Compared to the current generation of graphics cards, the GeForce 9800 GT is getting a little old. If you have cash, I would get a new one, such as the SAPPHIRE 100283-3L Radeon HD 5770 1GB
. If you want a little more power, the HIS iCooler V H583FN1GD Radeon HD 5830 1GB
could be consiered, however, it is a big card so make sure you have room for it and it is at the limit of what your power supply can handle.