in this model is the ram potentially upgradeable to 6GB?

According to EveryMac, your serial number is a 20" Early 2008 iMac:

Keywords=W8836042ZE2..Showing 1 - 1 of 1

iMac "Core 2 Duo" 2.4 20-Inch (Early 2008) 2.4 GHz Core 2 Duo (E8135)

Intro. April 28, 2008 Disc. March 3, 2009

Order MB323LL/A Model A1224 (EMC 2210)

Family Early 2008 ID iMac8,1

RAM 1 GB VRAM 128 MB

Storage 250 GB (7200 RPM) Optical 8X DL "SuperDrive"

This generation can be updated to 6GB PC2-6400 RAM (2G + 4G sticks). It's physically possible to install 2x 4GB sticks for a theoretical 8GB, but Intel's controlling chipset for the CPU won't acknowledge or use the last 2GB*.

iFixit has both chips in stock:

PC2-6400 2 GB RAM Chip

PC2-6400 4 GB RAM Chip

Installation de RAM dans les iMac Intel 20" EMC 2133 et 2210

If you go looking for this RAM on the used market, be careful to look for PC2-6400 SODIMMs, which are much less common than the slower PC2-5300 SODIMMs and the faster PC3-8500 SODIMMs. The PC2-6400 generation was pretty short; it was used only in the Early 2008 iMacs and the Early 2009 MacBooks, and wasn't around in the Windows marketplace for much more than a year.

As far as brands are concerned, I'm much less fussy than I used to be. Samsung chips are normally regarded as the gold standard, but a lot of RAM companies use Samsung chips. When you're buying new RAM, you should always buy from a retailer that offers a lifetime warranty; RAM does die occasionally, and it's better to have a commitment to replace. Brands that you typically see installed as OEM by Apple with Apple's stickers on them are Samsung, Micron, Hynix and Elpida; these sticks are available rebranded from all sorts of retailers.

I tend to buy my new RAM from OWC in Illinois; they've been a Macintosh-market retailer and manufacturer for nearly 30 years, so I can be reasonably confident that they'll be around long enough to answer a warranty claim - they're not going to disappear before the "lifetime warranty" has gotten close to what I consider an appropriate length. They have excellent customer service, and the people who answer the phone are tech-savvy.

*Computer industry fun fact: The Intel memory controller limitation was universal in Core 2 Duo computers of the era, but most of the big manufacturers (Dell, Sony, HP, Gateway, Toshiba, Acer etc) were perfectly happy to sell you 8GB of high-priced RAM, even though the last 2GB were a total waste of money. Apple was the one major manufacturer that refused to offer the useless expensive RAM as a build-to-order option.