Looking to upgrade

Looking to upgrade my older pc. I'm not a gamer, but I am running a ceton infinitv cable card tuner, so looking to get a performance boost. Would like to keep it under 400. Here is my current config:

os- windows 8 pro

cpu- amd phenom II x4 940 3ghz

motherboard- ASUSTeK Computer INC. M3N78-VM Rev X.0x
Serial Number: MF7095G03002354
Bus Clock: 200 megahertz
BIOS: American Megatrends Inc. 1801 09/10/2010

video- NVIDIA GeForce 9800 GT (512mb)

drives- Maxtor 6V300F0 [Hard drive] (300.09 GB)
        WDC WD10EADS-00L5B1 [Hard drive] (1000.20 GB) 2x

memory- slot 1- 2gb ddr2..micron pc2 6400 (400mhz) 
        slot 2- 2gb ddr2..micron pc2 6400 (400mhz)