I did, I've been there before I went to these forums, that's where I was planning to upgrade my memory. My question was, why is it that my computer can run all the memory downgrading it to 533MHZ but it can't run the memory without the 533Mhz that downgrades my 800Mhz, not what kind of memory I need lol. And if my computer can't handle the 800 Mhz then why all upgrades with 800 Mhz? that's like a waste of Mhz ya know lol.
Anyway, here is my DxDiag, so may be that might help out to answer the question.

Operating System: Windows 7 Ultimate 32-bit (6.1, Build 7100) (7100.winmain_win7rc.090421-1700)
Language: English (Regional Setting: English)
System Manufacturer: Dell Inc
System Model: Dimension C521
BIOS: )Phoenix - Award WorkstationBIOS v6.00PG
Processor: AMD Sempron(tm) Processor 3400+, ~1.8GHz
Memory: 2560MB RAM
Available OS Memory: 2558MB RAM
Page File: 789MB used, 4326MB available
DirectX Version: DirectX 11
DxDiag Version: 6.01.7100.0000 32bit Unicode
Card name: NVIDIA GeForce 8600 GT
Manufacturer: NVIDIA
Chip type: GeForce 8600 GT
Display Memory: 1525 MB
Dedicated Memory: 501 MB
Shared Memory: 1023 MB
Current Mode: 1280 x 1024 (32 bit) (60Hz)
I think that's all the info you need to know, if there is something else, please tell me. Thanks.