I'm also using a venerable XPS 420 (Q6600, 120MB SSD, 4GB DDR2 667MHz Ram, ASUS GTX550Ti, + various HDD's) and upgrade the OS from Vista Home Premium 32Bit to Windows 8.1 Pro 64 Bit. The SSD and OS upgrade made a huge difference. No possibility to upgrade the BIOS as DELL's motherboard is proprietary (0TP406, though X38 based, is a BTX board), with a factory LOCKED Bios. The 64 Bit OS, however, is the most important step to making best use of the 8GB RAM max limit on this system. I suppose I should upgrade the computer to 4x2GB 800Mhz DDR2 RAM, but I'm not sure I want to buy DDR2 these days. At this stage, I'm contemplating doing one last project for "fun". The pin mod to OC the Q6600 by tricking the system into thinking the CPU is a faster part than it actually is. If a bump up to 3.0Ghz makes a difference...maybe I'll buy the faster RAM too. This would also force me to take out the shroud/cooler and replace the thermal paste (for the first time). In any event, I'll just run the system awhile longer and then move to strip the viable components for reuse in a self-built desktop. Frankly, as I'm not a heavy gamer, the current setup is doing AOK for me...the upgrading project is just for fun.
This DELL was a decent purchase, considering how long it's been running. That being said, I will never purchase another desktop with a proprietary MB and locked BIOS. Never. Not when you can build, or have built on your behalf, a much higher performing and OPEN specified system that can support significant upgrades.
I bought my Dell system for ~ USD $3000 while working in Malaysia. At that time, I could've delivered a generic specification to a local IT shop and had a hi end gaming system for the same amount. Live and learn.
PS: The power supply was rated for a resource intensive, power sucking, 8800GT self contained oven. It's still marching along quite nicely with a newer, cooler, much more capable and power efficient card. DELL calls it a 425W unit...but its' peak load is actually much higher than that. I suppose I could've gone for a more powerful card...but the GTX550 Ti is OK for my needs.