so the +12V Rail only has 18A? is there only one +12V or is there something like +12V1 & +12V2?
if there is only a single +12V and putting out only 18A, you'll want an upgraded PSU as well.
18A is only 216W, and on a 500W PSU thats pathetically low.
all Video cards use the +12V rail on a PSU, most current PC's as well for the last half decade also mainly use the +12V (motherboards, which then divert power to the CPU, RAM, HDD, etc) and only tap into the 3.3 and 5V Rails as needed.
but since the Video card uses the 12V rail, it mainly sucks most of the power the system has to use.
Many PSU's can not use the maximum values for 3.3, 5 & 12V all at once, as most have a combined maximum output to their rated value.
I just added up the totals too, the 3.3, 5 & 12V values listed above don't even max out at 500W, your looking at 445W or so max.
Galaxy makes some decent Video cards, but I'm unsure of their PSU's and quality.
If it were me I'd consider replacing the PSU as well, surprised the 9800GT ran in that system as long as it did.
since it draws 105W of power:
nVidia GeForce 9800 GT Video Card - Reviews, Specifications, and Pictures - GPUReview.com
Your missing a lot of other info asked for. The motherboard model isn't a gigabyte (Your missing a lot of the model numbers info between Gigabyte and SL2, you got the Brand name, and the last part of the Models designation but nothing else) SL2
and Socket 775 doesn't tell us anything about the CPU, only the socket design that the CPU fits in.
Download and run Speccy:
Speccy - System Information - Free Download
Let it analyze your system, and then highlight all the info on the main window, copy and paste it into your reply.
For example my system info:
Operating System
MS Windows 7 Home Premium 64-bit SP1
CPU
Intel Core i7 920 @ 2.67GHz 35 °C
Bloomfield 45nm Technology
RAM
6.0GB Triple-Channel DDR3 @ 534MHz (8-8-8-19)
Motherboard
MSI MSI X58 PLATINUM(MS-7522) (CPU 1)
Graphics
P244W (1920x1080@60Hz)
1024MB GeForce GTX 285 (nVidia) 45 °C
Hard Drives
244GB Seagate ST3250310NS (RAID) 28 °C
488GB Western Digital WD SATA 500GB (RAID)
488GB Seagate ST3500418AS (RAID) 26 °C
488GB Western Digital WDC WD5000AAKS-00YGA0 (RAID) 35 °C
Optical Drives
HL-DT-ST DVD-RAM GH22NP20 ATA Device
PIONEER DVD-RW DVR-216D
Audio
HTO STRIKER 7.1
Gives us an easier better way to see what your system specs are.