I have an old-school Dell Dimension 3000 which takes up to 2GB (2x1GB) of 184-pin 333 or 400MHz DDR RAM. I thought I read somewhere that my box needs non-ECC RAM, but I don't know if this is true, and if it is, I don't understand why. I just found some of what I need for cheap (not easy with such ancient stuff), but it's ECC. Can I use it? Thanks for your help!
No.
Well, technically it is usable, but most computers are going to be finicky when dealing with the extra bits of memory that are associated with ECC ram.
Just stick with non-ECC if you want to prevent yourself from having issues.
This is a link to a rather cheap 1gb module from OCZ, they make good RAM.
1GB (2 x 512MB) PC2700 DDR RAM Desktop MEMORY 333MHz single sided non-ECC, 1GB (2 x 512MB) PC2100 DDR RAM Desktop MEMORY 266MHz low density non-ECC, HP 18GB(18x1GB) DDR PC2100 CL2.5 ECC 266MHz REG SERVER RAM HP 261585-041, 1GB 1024MB PC3200 DDR RAM Desktop MEMORY 400MHz 3200 low density non-ECC, Hynix (6x1GB) DDR PC2100R ECC Reg CL2.5 HYMD212G726BS4M-H Server Ram, SUPERTALENT 8GB (4X2GB) PC2700 DDR333 ECC RAM MEMORY FOR SERVERS, Server RAM 8GB 4x 2GB FITS HP xw6200 xw8200 ECC REG PC2-3200R DDR2 240pin Memory, M393B5273CHO - Samsung 8GB (2x4GB) PC3-10600 DDR3 ECC REG 1333MHz Server RAM, M393B5273CHO - Samsung 16GB (4x4GB) PC3-10600 DDR3 ECC REG 1333MHz Server RAM, M391B5673EH1 - Samsung 8GB (4x2GB) PC3-10600E DDR3 ECC REG 1333MHz Server Ram, M312L5720CZ3 - Samsung 8GB (4x2GB) PC3200 DDR ECC REG Server RAM, HMT351R7BFR8A - Hynix 16GB (4x4GB) PC3L-10600R DDR3 ECC REG 1333MHz Server RAM, Server RAM 4GB 2x 2GB PC2-3200R 2Rx4 ECC Registered DDR2 400 240-pin DIMM Memory, 2GB (2 x 1GB) KINGSTON KTD8300/1G PC-3200 DDR 400Mhz PC RAM! Non Ecc!, Samsung 11GB (11x1GB) PC2-3200R DDR2 ECC M393T2950CZP-CCCQ0 Server Ram Memory, 4GB 2x2GB Memory for Sun Fire x2100 V440 V65x 184pin RAM ECC Registered DDR, 16GB 2x8GB ECC Registered DDR3 RDIMM Compatible with Sun Fire X2270 X4170 RAM, 4GB 2x2GB ECC Registered DDR RDIMM Compatible with Sun 440 DC AC DC RAM Memory, 8GB 2x4GB 240pin for Tyan Thunder n6650W Upgrade Memory RAM ECC Registered DDR2, 4GB 2x2GB Memory RAM for Tyan Thunder n6650W 240pin ECC Registered DDR2 RDIMM, #74 LOT[100]: Micron PC2700 512MB RAM ECC Server memory DDR-333, HYNIX 2GB (2x 1GB) PC2 6400S DDR2 800MHz NON-ECC LAPTOP NOTEBOOK RAM MEMORY, Samsung 256MB DDR 266mhz PC2100 NON ECC Unbuffered Desktop RAM M368L3223DTL-CB0, 16GB 2x8GB RAM for Acer Altos G540 R520 R720 240pin Memory ECC Registered DDR3, 14 Lot Samsung for HP 373030-051 PC3200R-30331-F3 14x2GB DDR CL3 ECC Server RAM, Server RAM 4GB 2x 2GB Kingston KTC-ML370G3/4G PC2100R ECC DDR 266 184-pin Memory, Lot of 4 Hynix 1GB 1Rx4 PC2-3200 Reg ECC DDR2 RAM HYMP512R724-E3 4GB Total, 4GB 2x2GB Memory RAM for Apple Power Mac M9590LL/A M9591LL/A M9592LL/A ECC DDR2, 4GB 2x2GB Memory for Gigabyte GA-4 GA-4MPSV 240pin RAM ECC DDR2 UDIMM New, 4GB 2x2GB Memory for Tyan i3100 i3100 I3100 240pin RAM ECC Registered DDR2 RDIMM, 4GB 2x2GB Memory RAM for Tyan Toledo i3200R i3200R i3210W 240pin ECC DDR2 UDIMM, Elpida 6GB (6x1GB) PC2 3200R 1Rx4 DDR2 ECC EBE10RD4ABFA-4A-E Server Ram Memory, 2GB 2x1GB Memory Compatible with Dell Dimension XPS XPSGEN5 240pin RAM ECC DDR2, 4GB 2x2GB RAM for Dell Dimension XPS XPSGEN5 Gen5 400 240pin Memory ECC DDR2, 2GB 2x1GB RAM Compatible with Intel Servers S875WP1-E CANTERWOOD Upgrade ECC DDR, 4GB 2x2GB RAM Compatible with Sun Blade X8440 Base Server ECC Registered DDR2, 4GB 2x2GB Memory Compatible with IBM System X x3250 240pin RAM ECC DDR2 UDIMM, 4GB 2x2GB RAM for SuperMicro P4 P4DLR P4DLR+ P4DME-M Upgrade ECC Registered DDR, 2GB 2x1GB ECC Registered DDR RDIMM for SuperMicro P4 P4DE8 P4DEE P4DEI RAM, 2GB 2x1GB RAM for SuperMicro P4 P4SCT+ P4SCT+II 184pin Memory ECC DDR UDIMM New, 2GB 2x1GB Memory RAM for SuperMicro P4 P4SC8 P4SCI P4SCT 184pin ECC DDR UDIMM, 2GB 2x1GB Memory RAM Compatible with SuperMicro P4 P4SDM P4SDA+ P4SDM ECC DDR, 2GB 2x1GB RAM for SuperMicro P4 P4SAA P4SDA P4SDA+ Upgrade ECC DDR UDIMM Memory, 4GB 2x2GB RAM for IBM ThinkCentre E30 240pin Memory ECC DDR3 UDIMM New, 8GB 2x4GB Memory Compatible with IBM ThinkCentre E30 240pin RAM ECC DDR3 UDIMM, 2GB 2x1GB RAM Compatible with Tyan Thunder K7X K7X GC-HE K7 ECC Registered DDR, 8GB 2x4GB RAM for SuperMicro H8D H8DMT-IBX H8DMT-IBXF ECC Registered DDR2 RDIMM, 2GB 2x1GB Memory RAM for Toshiba 3300 3300R 3310 184pin ECC Registered DDR RDIMM, 4GB 2x2GB RAM for Fujitsu PRIMERGY J510 TX100 Upgrade ECC DDR3 UDIMM Memory, 4GB 2x2GB Memory for Fujitsu PRIMERGY RX100 TX120 S3 TX140 240pin RAM ECC DDR3,