Q: snow leopard to el capitan: over present system or clean install?

So, should I install El Cap over my present system

Yes, If it was me I would install the upgrade over your present OSX.  As long as you are running 10.6.8

El Capitan is going to be happiest with a minium of 8 GB of RAM.

It is a normal OSX upgrade like any other. In this way you pick up right where you left off.

Always prudent to have a back up plan in place.

I prefer

get the installer and commit to media, I prefer Apples method because posts on here show users who tanked something with an article they found elsewhere.

Create a bootable installer for OS X - Apple Support

backup. this is critical. I use Time Machine

Use Time Machine to back up or restore your Mac - Apple Support

then taking inventory of my software and serials and accounts and getting the installers handy, verify they will work with the developers because sites like "roaring apps" are subject to the opinions of the internet (e.g Warcraft III does not run in 10.8, World of Warcraft Does, different game entirely but some confusion on that site)

shut down, detach external devices

run the installer

then wipe the drive

then clean install with no other devices present

then run the App Store updates for any additional possible apple updates

then re attach other devices if I need them.

then reinstall my 3rd party stuff clean from archived installers or new one on the web

then get the data from the backup.

more steps but I spend less time troubleshooting odd ball residual results and residual performance issues if they happen.

Generally speaking you should always upgrade to the latest possible system that you meet the specs for ,as long as you have no “must have” apps that do not meet the specs. https://support.apple.com/kb/SP728?locale=en_US

Several things you should do before upgrading…

1. If you have issues on your present system don't compound them by installing over them. Solve them first then upgrade.

2 Check HDD for free space. At least 8.8GB of available storage. Preferably much more.

3. Check your applications for compatibility here…http://roaringapps.com/apps

4. Make a bootable clone SuperDuper! or CarbonCopy Cloner or use TimeMachine.

5. Disconnect all peripherals, including USB hubs.

6. After downloading the installer, it will be in /Applications folder. Duplicate it and then drag the copy it to another locations like Downloads, if you want to save the installer.

7. Be patient, the download takes a long time and the install may look like it has died at times. Ignore it.

*Also how much RAM do have? 2GB is stated minimum but 4GB is more realistic.

Unless you're having problems with your current system and setup I would install El Capitan over what you currently have. 

I agree with leroydouglas' comment about EC and RAM:  upgrade to 8 GB  if at all possible.  If you increase your RAM I recommend getting it from Crucial.com or otherworldcomputing.com. 

Also I second machack's suggestions of things to do before upgrading.

I have purchased RAM several times from Other World Computing and have always been very satisfied with the product and service. They have on-line instructions on how to replace the RAM. OWC has also tested RAM above what Apple states is the maximum. I now have 6GB installed on a early 2008 iMac supposedly limited to 4 GB and noticed an improvement.

Crucial

Other World Computing 

Crucial and OWC and it seems Crucial is about half the price. Is the RAM from the two outfits equal in quality?

Yes

Yes.  I've purchased all of my RAM from Crucial.