You will probably do better selling your whole system and buying the parts for a new one. That's not as wild as it may sound. Your whole working system is probably worth more than the sum of its components, and you can build a much better one for very little money.
For example, if you live anywhere near a Fry's Electronics, they often have great inexpensive combos. I just checked, and they don't have much in the way of a motherboard/CPU combo, this week, but you can start by getting
512 MB PNY PC3200 for $19.99 after mail-in rebate.. They regularly have combos like a Sempron 2800+ with an ECS motherboard for around $80, but you have to get them when they have the decent boards. The full sized boards with nVidia and VIA chipsets are great. They won't be your favorite for overclocking, but they'll be so much faster than what you're running, now, you won't worry about it.
As another example, last week, they had WD 120 GB ATA 100 drives for $20 after rebates. This week, they have
120 GB SATA drives for $49 after rebates.
If you're not within range of Fry's, set a budget (including what you can get for your old system), check other stores for good values, and enjoy a complete setup that will rock for some time to come.
