Is it worth upgrading to 4GB DDR3? (From 2GB DDR2)

you can download something like CoreTemp which has a RAM usage monitor... then you'll know if it's being used 100%

what's in it now... i presume a single 2gb stick? then hop on ebay and get another 2gb stick... i dont thing you need to buy a kit.

of course a reformat and fresh install will help as well

There are very few motherboard that support ddr2 and ddr3(and none that support them at the same time). They were all retail boards(had 2 slots for ddr2 and 2 for ddr3). I have never seen or heard of an OEM board that did this, but that doesn't man one doesn't exist. We would need the make and model of the system or the board to know for sure, but most likely your board does not support ddr3. Adding another 2Gb stick of ddr2 may help.

If your goal is to use this computer as best as it can while using as small amount of funds, increasing the amount of ram and potentially upgrading its boot hard drive to a 120GB Solid State drive are incredibly cheap methods to improve performance. If you intend to keep the computer, upgrading the total ram to 4GB is a large help. Getting above the 2GB mark helps with keeping multiple programs open and to increase the machines ability to use multiple windows and multiple browsers. The first thing you want to check is to see if your system has a set of 2x1GB sticks or 1x2GB stick. If its 2x1GB sticks, purchase the one from the link below.

Here is a set of 2x2GB sticks. $79.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231166

As far as getting a better boot drive, the 120GB SSD that I was talking about you should try to find something around the price of $70. Most SSDs are fairly identical, as they run so incredibly fast, you arn't looking for the best performance possible, but the best price possible. Doing this will increase the boot time of the machine and the load time of all programs on it. It makes your machine lightning fast. As most people who are only doing general browsing dont really need high end processing, a good SSD gives the snap back to a machine to make people happy with it. The feel of a machine is generally more important than true processing performance for home users.

Here is a cheap 120 GB SSD for $59.99 at the time of posting.
http://www.newegg.com/Product/Product.aspx?Item=N82E16820178455

Remember, you want to reinstall windows(or whatever Operating System you have) on this SSD. The other hard drive you have can have its important documents etc backed up off the drive, you can then format the drive and use it as a data drive on the computer. This will give you extra space as well. Reinstall the major programs to the SSD but keep all your data files on the HDD. Alot of people talk about SSDs getting bogged down by too many writes to it, but that matters not anymore as this problem has been fixed with hundreds of TiBs being written to the SSDs without corrupted sectors occuring.

If you want to take it as far as you can, upgrading the video card is a possibility as well. But that all depends on what slots the motherboard has as it is a few years old, so upgrading the computer's video card is not recommended. But the SSD is something that can carry over when you want to purchase a new computer down the road, so that is something I highly recommend.

Trying to upgrade your computer's ram to DDR3 would be a waste of funds as it would require purchasing a new motherboard and with that potentially a new processor. When you upgrade a desktop computer, if you want to upgrade ram from DDR2 to DDR3, the motherboard and processor come with it. The motherboard and processor are two pieces literally built for one another. With that the motherboard has certain standards of slots that are built in. RAM's DDR, PCI's express, USB's, and SATA's slot versions are the things to look at as they are what are upgraded to keep to the latest standard. But I assume you want to keep the rest of the machine intact and keep costs as low as possible. So forget trying to upgrade everything else unless you want to purchase a whole new machine.