My comp has 4 slots. Two for SDRAM and two for DDR. I currently have the SDRAM filled with two PC100 DIMM @ 256 making my sys total RAM to 512.
Questions are:
Do I only have to use PC100? How do I know.
Am I only supposed to use one (DDR) or the other (SDR)?
Which is better? I am told DDR…why?
I have other questions but this is a start I guess. I would appreciate any help you all can offer because my comp techno skills are limited in hardward.
Thanks.
Hey MTB
DDR stands for dual data rate and is faster than regular SingleDR. The actual speed increase is most likely not twice as fast because of latency; actual speed will depend on your motherboard and other things.
I doubt you can run SDR and DDR at the same time unless your motherboard specifically supports it (I’ve never heard of this before but it would be best to read your mobo info).
DDR is, for the moment, the most common and cheapest RAM so it is probably more future proof. Note that there are different sorts though classified by speed resulting from how many Hz each can stand (two naming conventions make it confusing, but you might see numbers like DDR266, DDR333, DDR400, or PC2700). Again, consult your mobo info to see what sort(s) of DDR it supports - I think it is OK to use higher spec DDR in lower spec systems (you just won’t be getting the best out of it, although buying higher spec will again be more future proof).
As you are probably going to have to ditch your SDR to upgrade either way (as you likely can’t run SDR and DDR at the same time), DDR is probably the best bet.
Hi MTB!
What I’ve been told is that you should only use one sort of memory, sdram or ddr. Don’t know why you have both, never heard before.
And for that DDR question is, it’s faster memory than sdram.
If you have your motherboard manual left you could check right answers from there or you could look from internet that manual if you only know type of your motherboard.
Cheers, Jimking 
Thanks for the help and info guys.
Aside from getting two ddr 512 sets…I need a new computer. =(
I just bought an HP 3.0E Ghz PIV system that rocks! It only cost $860 bucks too. DVD writer, 160 GB HD, and a whole lot more! And to top it off, I got it at Wal-Mart of all places! It’s actually cheaper there than it would have cost me to get it from HP themselves!
Anyway, just wanted to let you know that there are some cheap (as in monetary) units out there. Just do some looking around. I’m not one of those people who like to upgrade a lot, and besides, for another $800 sometime in the future I’m sure I can buy a whole new system again, so who needs to upgrade?
Oh yeah, my system has 512 MB DDR RAM (PC3200), and I’m able to work on fairly large models,1-2 million easily. It starts to write to the hard drive, Page files I think it’s called on a PC, higher than that, but it’s still not that bad. I’ll upgrade my RAM when I need to.
Good luck on your search!
Also, check out your motherboard manual (if you have it). Sometimes reccomendations are in there.
Have three working computers.
Two old, one new.
Know next to nothing “under the hood”.
Last year had a little money and decided to
upgrade ram.
So much for that which you couldn’t care less.
Go to www.belarc. com.
This is a store that sells computer memory and parts cheap.
It puts a small program on your computer that tells you what you have and what you can use.
You don’t have to order to use it.
I did and the prices are competitive with other places.
I ordered a video card and memory from them.
With what I paid the technician to install the video card I still came out ahead.
Go to the site, download the program, it works and is on the up and up.
Luck
edit :one of the computers was 6 years old from an manufacturer that went out of business, they had the specs on the motherboard
something wrong with link,you will have to type what you see.
That prog gives a nice lot of info 
Doesn’t give max poss RAM tho.
You put a space in the URL, bic’.
angstrom, thank you, I was beginning to think
the post was a waste.
Sorry about the link, this is the first time
I ever tried it.