here is my info...
Computer System
Manufacturer: Dell Inc.
Model: OptiPlex GX280
Operating System
Microsoft Windows XP Professional (Service Pack 3)
Version: 5.1.2600
Build Number: 2600
Install Date: 15/12/2008
Install Directory: C:\WINDOWS
Locale: English (en) - United States (US)
DirectX version: DirectX 9.0c (4.09.0000.0904)
Motherboard
Manufacturer: Dell Inc.
Model: 0G8310 (v )
Bios
Manufacturer: Dell Inc.
Model: A04
Release Date: 09/02/2005
Processors
Intel(R) Pentium(R) 4 CPU 3.00GHz (GenuineIntel)
Max clock speed: 2.99 GHz
Socket: Microprocessor
Version: Model 4, Stepping 1
Memory
Total Current Physical Memory: 1.99 GB
Total Current Virtual Memory: 2.00 GB
Maximum Memory Capacity: 1.00 GB
Memory Devices:
Type: SDRAM - Speed: 533 MHz - Capacity: 1.00 GB
Type: SDRAM - Speed: 533 MHz - Capacity: 1.00 GB
Video Devices
Intel(R) 82915G/GV/910GL Express Chipset Family
Resolution: 1280 x 1024
Colour Depth: 32 Bits
Refresh Rate: 60 Hz
Intel(R) 82915G/GV/910GL Express Chipset Family
Resolution: x
Colour Depth: Bits
Refresh Rate: Hz
Sound Devices:
SoundMAX Integrated Digital Audio
Physical Disk Drives
WDC WD800JD-75MSA3
Size: 74.50 GB
Number of Partitions: 1
Volumes
Logical Disks
C:
Disk Type: Local Fixed Disk
File System: NTFS
Capacity: 74.50 GB
Free Space: 9.50 GB
D:
Disk Type: CD-ROM Disc
File System:
Capacity: 0.00 GB
Free Space: 0.00 GB
Page Files:
C:\pagefile.sys
Initial Size: 1.49 GB
Current Size: 1.49 GB
Maximum Size: 2.98 GB
File System: NTFS
Optical Drives
HP DVD Writer 740b - D:
Printers
Microsoft XPS Document Writer (port: XPSPort
HP Deskjet F2200 series (port: USB001)
Network Adapters
Broadcom NetXtreme 57xx Gigabit Controller - Packet Scheduler Miniport
IP Address: 192.168.XXX.XXX
Default Gateway: 192.168.1.1
Subnet Mask: 255.255.255.0
Primary DNS: 208.180.42.100
Security Software
Anti-Virus
avast! Antivirus 5.0.83886757 (AVAST Software)
Firewall

i want to be able to play games like sims3 and WOW...

Well, according to this C|Net Review, your Dell is using a PCIe (PCI Express) x16 card. This means you can replace it with another PCIe card which gives you lots of options.

But, there are a couple issues. First, most factory made PCs come with power supplies that are barely adequate for the hardware they come with. Graphics cards are often the most power hungry devices in a computer so it is common to need a more powerful supply when upgrading the graphics card. You need to determine the specs of your current supply (typically, there's a label you can see when you open the side panel). Then note the power requirements of the new card and ensure your current supply has the horsepower (and connections) to support it, or plan on buying a new supply too.

You told us your intended use and that's great, but budget is also necessary. I recommend you set a budget (but don't forget a new power supply) before shopping about. Normally, I would post a link to Tom's Hardware Best Graphics Cards for the Money - November 2010 (December's list is not out yet) but just yesterday, they did an article on cards for playing WOW. See Tom's Hardware Performance Guide for WOW - 24 Graphics Cards.