I don't know, I've just spent about 10-15 minutes researching your system and from what I've seen, it may be "unlikely" your current Dell Dimension E520 is up to the job. I'm currently using an old Dell Dimension 5100c, some of my spec's look a lot like yours, and if it were me, I wouldn't try to run it! You've got to understand something, our older units were basically designed for what was available during the time period of their construction. If we were talking about playing something like Unreal Tournament 1999 or maybe Deus Ex or maybe Quake III Arena or the first and second installments of Half Life, then yeah, we could play those on our computers all day long. But when your venturing into the world of modern day gaming, the new games of today take little notion of being anywhere close to "backwards compatible". Today's games, I hate to say, if they could run for an instant on our older unit's, would rip them apart, freeze up, and react so badly, it would just piss us off!
Perhaps if modern day gaming is something that you want to venture into, than I would seriously recommend either upgrading everything there might be inside you computer to upgrade. Drop the on-board video adapter off the mother board and get yourself a decent little PCI-e video solution. Maybe dump the on-board basic sound controller and get a decent Creative Labs sound card, like one from their "Ti" series. Then I see where the computer supports upgrades from the Pentium 4 to a Core Duo, maybe get the dual core thing, for the Pentium 4 will not do the trick. And do as I did on my 5100c, and that was upgrading it to the max on RAM, yours and mine can be upgraded to a max of 4 Gig's total, if your dancing around with just 2 Gig's of RAM, that would be an upgrade I would put almost first on the list of things to do.
Or we can just go ahead and scrap the old E520 and think about purchasing something new! So one way or the other, upgrading your old system or buying something new, one way or the other I'm sure you'll figure out a way to do some modern day gaming!