No point in upgrading that machine. The Intel Celeron CPU is absolutely abysmal and will bottleneck any modern graphics card horribly, and you're not really in a position to upgrade it either, since it's on an old socket type (LGA 775) and frankly put, it'd cost you less just to buy a new motherboard and CPU instead of trying to upgrade it. I could easily find you a new motherboard and CPU combo that would work, but judging from your current budget, I'd say it would be kind of hard to find one that wouldn't leave you broke. The best I could do that wouldn't be absolutely terrible is around $200, and that doesn't include the price of a newer graphics card (eg the 6850, don't even consider something like the 9800GT, it's very dated), a new power supply to drive said graphics card, and a new case, assuming your current one is a pre-built.
Wait until you have the cash for a new computer.