First off, before you do any upgrading, upgrade your power supply. You don't say what your video card is, but even if it's a basic or onboard, 450 watts is cutting it pretty slim, even accounting for a name brand. 450 watts is fine for what you're doing currently.
I'd recommend a Corsair 550/600 watt unit just to give yourself some breathing room.
Newegg.com - CORSAIR Builder Series CMPSU-600CX 600W ATX12V v2.3 Active PFC Power Supply
Your motherboard is a Socket AM2+ motherboard, so you can upgrade to 16GB of RAM max, but the sweet spot right now is 4-8 GB depending on use. For you, 8GB would probably be ideal.
I live pretty close to the Mushkin plant, so I tend to recommend them over most other brands, but a 4GB x 2 kit should be fine. Pick any one that looks price friendly to you. Link here if you need feedback
(x 2, they don't have any DDR2 kits that are 4x2 on Newegg atm, other vendors might offer them, otherwise you'll have to buy them one stick at a time from the same family)
Your CPU is also relatively low end. (Ironically enough, I'm upgrading the 4600+ I have this week). Since you've got an AM2+ motherboard, you can pick either an AM2+ CPU or an AM3 CPU. If you're looking for ultimate power at a decent price, look at the Phenom II X6 CPUs - the 1055 should have plenty of horsepower for what you're doing and will be a huge step up from what you have now.
Newegg.com - AMD Phenom II X6 1055T Thuban 2.8GHz 6 x 512KB L2 Cache 6MB L3 Cache Socket AM3 125W Six-Core Desktop Processor HDT55TFBGRBOX
since I don't know what video card you have, you'll have to reply back and let us know.