Hi im new to this so please bare with me lol. I av to jst built a new PC, the spec is

Pentium 4 processor 478
160Gb harddrive
2Gb RAM
Abit-VT7 Motherboard
Radeon 9550 256mb
400 watt power supply
Windows Xp Home Edition

When i play games, (rise of the nations, empire earth 2) within 15 mins of the game the screen freezes then goes on power save. My PC wont respond at all and i have to reset the PC to get back on. I sometimes get msgs like Your graphics accelerator has been reset. Its ok when i play games with basic graphics such as football manager. I am thinkin of gettin a new graphics card but might not because iv heard other people still get it with new carsds. I also dont know much about PC's so wouldnt know how to test the temps. If this helps when the PC boots up sometimes the Windows Xp logo is crackly. I av also tried formatting the PC and new drivers but nothin has happened. It only seems to happen when the screen starts to get crowded in the games. Any suggestions? Thanks