Budget Upgrade for my 2004 Dimension 4700

High im a regular lurker first time poster and the reason im here is to ask about upgrading my old Dell Dimension 4700. The components are pretty standard and also out dated here they are:

General
Processor P4 Prescott
Processor speed 3000 MHz

Memory
Amt of RAM 512 MB (max 2048 MB)
RAM type DDR2 SDRAM

Motherboard
Motherboard chipset Intel 915G

Graphics
Graphics card ATI Radeon X300 SE
Amt of video RAM 128MB

Storage
Hard drive 160 GB (7200 rpm)
Interface Serial ATA
Other drives included DVD/CD-RW combo drive

Modem and Networking
Modem included? 56k PCI data/fax modem
Networking Integrated Fast Ethernet 10/100

Expansion/Architecture
PCI slots (free) Total: 2 PCI slots, 1 PCIe x 1 Slot, 1 PCIe x 16(graphics) Slot
Ports 8 x USB 2.0 ports (2 front, 6 back), DVI out, VGA out, TV-out, Ethernet, modem, front headphone jack, serial port, parallel port, 2 x PS/2

it's a standard home model thats showing its age and has slowed considerably and I want to know if its possible to upgrade it to be more of a multi-media PC for school work with the possibility of casual gaming which include Starcraft 2, Diablo 3, WoW, Left4dead, half life Etc...

My expected budget is 800 dollars give or take barring school expenses and salary and I intend to keep the monitor, keyboard and mouse. Do you think you can help me or should I invest in a new computer?

P.S. I read the rules and am sure this is the right forum to put it in based off other questions I have found here, if not feel free to move it.0