Apparently the Altix 350 can be a little fussy about what RAM is installed, and I'm finding it difficult to identify known-working parts. A few searches of the forum going back to 2006 didn't add much. If you've got the part numbers for something you know works in these systems, please feel free to share it in this thread.
The same parts look like they should be compatible between the Altix 3000, 330, 350, and Prism systems...
WORKING
512MB: Micron MT18VDDF6472DG-335G2 (PC3200, x72, ECC, Registered)*
512MB: Samsung M312L6423ETS-CB3 - PC2700R-25331-Z (PC2700, DDR, CL2.5 ECC)
1GB: DATARAM 63674 / DTP63674B (01088204069)
1GB: Micron MT18VDDF12872-335C3 - PC2700R-25331-H0 (1GB, DDR, 333, CL2.5, ECC, REG)
2GB: DATARAM 63692 / DTP63692E A0 - DDR 184pin PC2700 ECC Registered
2GB: Smart SME72564FD8C0CGSG - 184P-DDR-2GB-PC2700 ECC-REG
NOT WORKING
1GB: gigaram GR75020-E1GB/333/MTL - 128MX72 PC2700 CL2.5 ECC, REG
1GB: Micron MT18VDDF12872G-335D3 - PC2700R-25331-J0 (1GB, DDR, 333, CL2.5, ECC, REG) - HP P/N 331562-051
1GB: Kingston KVR266X72RC25/1024 - 128Mx72 PC2100 (1GB, DDR, 266, CL2.5, ECC, REG
2GB: Elpida EBD21RD4ADNA-6B - PC2700R-25330-N1 (2GB, DDR, 333, CL2.5, ECC, REG) - HP P/N 331563-051
2GB: Kingston KTH-DL145/4G - 2GB, DDR, 333MHz, ECC, REG - for HP DL145 G2
2GB: Micron MT36VDDF25672G-335D2 - PC2700R-25331-Z (2GB, DDR, 333, CL2.5, ECC, REG)
2GB: Samsung M312L5720DZ3-CB3 - PC2700R-25331-F3 (PC2700, DDR, CL2.5, ECC) - HP P/N 331563-051
4GB: DATARAM 63716 - PC2700R (4GB, DDR, 333MHz, ECC, REG)**† (stacked chips on these DIMMs)
* Part number was included in a hinv posting from a working system; parenthetical bits from a web site
** Details from a printed tag from eCyclePro, claimed to have been tested with a RAMCHECK tester
† There's no documentation of 4GB DIMMs working in A350s, YMMV.
Edit: Added the DATARAM 63716 part.
The same parts look like they should be compatible between the Altix 3000, 330, 350, and Prism systems...
WORKING
512MB: Micron MT18VDDF6472DG-335G2 (PC3200, x72, ECC, Registered)*
512MB: Samsung M312L6423ETS-CB3 - PC2700R-25331-Z (PC2700, DDR, CL2.5 ECC)
1GB: DATARAM 63674 / DTP63674B (01088204069)
1GB: Micron MT18VDDF12872-335C3 - PC2700R-25331-H0 (1GB, DDR, 333, CL2.5, ECC, REG)
2GB: DATARAM 63692 / DTP63692E A0 - DDR 184pin PC2700 ECC Registered
2GB: Smart SME72564FD8C0CGSG - 184P-DDR-2GB-PC2700 ECC-REG
NOT WORKING
1GB: gigaram GR75020-E1GB/333/MTL - 128MX72 PC2700 CL2.5 ECC, REG
1GB: Micron MT18VDDF12872G-335D3 - PC2700R-25331-J0 (1GB, DDR, 333, CL2.5, ECC, REG) - HP P/N 331562-051
1GB: Kingston KVR266X72RC25/1024 - 128Mx72 PC2100 (1GB, DDR, 266, CL2.5, ECC, REG
2GB: Elpida EBD21RD4ADNA-6B - PC2700R-25330-N1 (2GB, DDR, 333, CL2.5, ECC, REG) - HP P/N 331563-051
2GB: Kingston KTH-DL145/4G - 2GB, DDR, 333MHz, ECC, REG - for HP DL145 G2
2GB: Micron MT36VDDF25672G-335D2 - PC2700R-25331-Z (2GB, DDR, 333, CL2.5, ECC, REG)
2GB: Samsung M312L5720DZ3-CB3 - PC2700R-25331-F3 (PC2700, DDR, CL2.5, ECC) - HP P/N 331563-051
4GB: DATARAM 63716 - PC2700R (4GB, DDR, 333MHz, ECC, REG)**† (stacked chips on these DIMMs)
* Part number was included in a hinv posting from a working system; parenthetical bits from a web site
** Details from a printed tag from eCyclePro, claimed to have been tested with a RAMCHECK tester
† There's no documentation of 4GB DIMMs working in A350s, YMMV.
Edit: Added the DATARAM 63716 part.