ok i bought some 512mb 184 pin dimm unbuffered 64mx64 ddr RAM, for my pc
i installed it alongside the 256 mb ram thats already in my comp. when i start my computer,my pc doesnt recognise the new ram. is there something ive doen wrong, or is it maybe the ram isnt compatible for my pc? if that is the case then how can i know what ram is right for me

i am on windows xp

Welcome to PCHF

could you please post some more details about your PC. Motherboard/PC make and model number would be a great place to start.

does this help?
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System Information
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Time of this report: 9/29/2005, 23:04:44
Machine name: FAMILY
Operating System: Windows XP Home Edition (5.1, Build 2600) Service Pack 2 (2600.xpsp_sp2_gdr.050301-1519)
Language: English (Regional Setting: English)
System Manufacturer: Packard Bell NEC
System Model: 00000000000000000000000
BIOS: Default System BIOS
Processor: Intel(R) Pentium(R) 4 CPU 2.40GHz
Memory: 256MB RAM
Page File: 259MB used, 347MB available
Windows Dir: C:\WINDOWS
DirectX Version: DirectX 9.0c (4.09.0000.0904)
DX Setup Parameters: Not found
DxDiag Version: 5.03.2600.2180 32bit Unicode

Display Devices
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Card name: NVIDIA GeForce 6600
Manufacturer: NVIDIA
Chip type: GeForce 6600
DAC type: Integrated RAMDAC
Device Key: Enum\PCI\VEN_10DE&DEV_00F2&SUBSYS_00000000&REV_A2
Display Memory: 128.0 MB
Current Mode: 1024 x 768 (32 bit) (85Hz)
Monitor: Plug and Play Monitor
Monitor Max Res: 1600,1200
Driver Name: nv4_disp.dll
Driver Version: 6.14.0010.7801 (English)
DDI Version: 9 (or higher)
Driver Attributes: Final Retail

Have you tried replacing the 256 meg stick with the 512? If that works, then means the 512 meg stick is not the same speed as the 256 meg stick..Or timing

Truely speaking never seen anyone place a 512 with a 256 Ram together. Usually most people use identical pairs of ram, this eliminates many problems. Maybe the problem is due:
- To different manufactures (ram)
- Also that a 256 ram cannot be put together with a 512
- You bought the wrong type of ram
hmmm.not really sure ed:

ps: check in the motherboard manual, it schould show a list of compatible rams and ram settings

Well just to make sure only stick the 512stick onto your mainboard, if it's recognized there's nothing wrong with that stick.
Putting 2 sticks together, always put the biggest in the first bank, they are most time numbered DIMM0, DIMM1 or that sort.
It could be as the_machine mentioned that your 2 sticks are of different speed and those 2 will not work together.