Advices on mobo upgrade need!

Hello everyone.
I have a AMD Athlon 4600+ Dual Core CPU with an ASROCK motherboard that I dont have much info. I also have a 8800GTS 320M. Some games are running a bit slow right now like Just Cause 2.
I want to upgrade my video card to a HD 5850 and wonder if it's possible to just upgrade the video card and keep the rest?

I found my motherboard info:
A780GXE/128M
Support for Socket AM2+ / AM2 processors: AMD Phenom™ FX / Phenom / Athlon 64 FX / Athlon 64 X2 Dual-Core / Athlon X2 Dual-Core / Athlon 64 / Sempron processor
Supports CPU up to 140W
Solid Capacitor for CPU power
ASRock DuraCap (2.5 x longer life time)
Supports Hyper-Transport 3.0 (HT 3.0) Technology
AMD 780G + SB700 Chipsets
AM3 CPU Ready!
FSB 2600 MHz (5.2 GT/s)
Supports Dual Channel DDR2 1066/800/667/533 (4 x DIMM slots) non-ECC, un-buffered memory, Max. capacity up to 16GB
Integrated AMD Radeon HD 3200 graphics, DX10 class iGPU, Pixel Shader 4.0, Max. shared memory 512MB
Integrated 128MB side port memory for iGPU
Supports ATI™ CrossFire™ and Hybrid CrossFireX™
2 x PCI Express 2.0 x16 slots (green @ x16 mode, blue @ x8 mode)
6 x SATAII 3.0 Gb/s connectors, support RAID (RAID 0, RAID 1, RAID 10 and JBOD), NCQ, AHCI and "Hot Plug" functions
1 x eSATAII 3.0 Gb/s connector
Supports 2 x IEEE 1394 ports (one port on back panel, one header on board)
PCIE Gigabit LAN 10/100/1000 Mb/s
Dual VGA Output: Support DVI-D and D-Sub ports by independent display controllers
Supports HDCP function with DVI-D port
Supports Full HD 1080p Blu-ray (BD) / HD-DVD playback
1 x USB/WiFi header
Supports Instant Boot
Supports Smart BIOS, ASRock OC Tuner, Intelligent Energy Saver, Extra ESD Protection
Free Bundle: 1 x ASRock XFire Switch Card
7.1 CH Windows® Vista™ Premium Level HD Audio (ALC888 Audio Codec)
Windows® Vista™ Premium 2008 Logo Ready
I/O Panel: 1 x eSATAII Port, 1 x IEEE 1394 Port, 1 x RJ-45 LAN Port with LED, 6 x USB 2.0 Ports

Without knowing your motherboard make and model, we can't say with 100% certainty.

You should have no issues doing so, specially if your PCIe is a PCIe 2.0 x16. If it is just PCIe x16, you might bottleneck your GPU slightly but still will be much more powerful than the 8800GTS 320M.

You may want to check your Power Supply Unit, to see if it can handle the 5850.

The video card should list power requirements.

I have an Antec 500W PSU that I purchased 1 year ago. I wonder if it will do. It was suppose to be a good one.

Here are the system requirements for the 5870:

ATI Radeon™ HD 5870 System Requirements

PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended (600 Watt and four 6-pin connectors for ATI CrossFireX™ technology in dual mode)
Certified power supplies are recommended. Refer to http://ati.amd.com/certifiedPSU for a list of Certified products
Minimum 1GB of system memory
Installation software requires CD-ROM drive
DVD playback requires DVD drive
Blu-ray™ playback requires Blu-ray drive
For an ATI CrossFireX™ system, a second ATI Radeon™ HD 5870 graphics card, an ATI CrossFireX Ready motherboard and one ATI CrossFireX Bridge Interconnect cable per graphics card (included) are required

The recommended PSU is for the entire system, so, ur PSU can support a single 5870.