What do you want your computer to do better? If your computer slows to a crawl when you have 10 web browsers open, you should just upgrade to 3GB of RAM (it's really cheap, you should probably just do that anyway). If it takes forever to boot up or start applications, you should get an Intel X25-M SSD as your system drive--I did this recently, and I could not believe the difference it made. If you want to play FEAR 2, you should upgrade to a newer graphics card; this $80 card is one I'm considering because it doesn't require external power, runs quiet, and will run Left 4 Dead at 1920x1200 at 40fps--you'll never see that performance from an 8300.

That system looks a little outdated. Might be tricky to upgrade it with just 300$. Unless there is something really specific that you want upgraded, I'd recommend a bigger budget.

$300 is more than enough to get a new cpu, mobo, ram, and video card.

Here's an AMD/Nvidia combination for ~255 w/ shipping:

1. BFG Tech BFGE96512GTOCBE GeForce 9600 GT 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported Video Card - Retail - $80

2. G.SKILL 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model F2-6400CL5D-4GBNT - Retail - $43

3. AMD Athlon 64 X2 5600 Brisbane 2.9GHz Socket AM2 65W Dual-Core Processor Model ADO5600DOBOX - Retail | BIOSTAR A760G M2+ AM2+/AM2 AMD 760G Micro ATX AMD Motherboard - Retail - $118

Now, you could shave off $18 and go with a weaker more energy efficient, but weaker CPU, but I decided not to skimp on that. The combo for that is located here though if you are interested.

Since there is about $45 to spare, that means you could push to the 300 limit and get a 9800 GT card as well if you wanted. NewEgg doesn't sell 8800GTs anymore so you are pretty much stuck with the 9X00 series.

XFX PVT98GYDLH GeForce 9800 GT 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported Video Card - Retail - $128 (and comes with a $30 MIR)

So, with shipping, you can get a 2.9 GHz AMD Dual Core CPU + new motherboard + 4gb ram + 9800GT 512mb video card for right at $302. You also room to downgrade to a cheaper 9800 GT brand (I only purchase XFX, but you can save some going cheaper) or even to a 9600 GT series to save even more but sacrifice gaming performance (9600 is certainly a nice upgrade from what you have right now though, so no worries there). Most of the additional cost of a computer lies in the things you already have, case, hd, psu, optical drive, those things all add up!

All you have to do is make sure this stuff will actually fit in your current case and you should be good to go. Of course, I can't guarantee all the products suggested will work together, so it will be up to you to determine if they will and spend your money accordingly. I did however, only post parts I would buy with my own money if I were in your situation. The only difference is, I'd had paid a little more for a Gigabyte motherboard as that's the brand I use and have never had problems with.

One last very important thing, your current PSU might not be reusable, so if that's the case, you will need to pluck down another $40 or so to get one compatible with this newer AMD board. In that event, you will want to go with the cheaper 9600 GT so you can fit in a decent PSU to power your parts. I only buy Rosewill, so here's one that should work and won't break the bank. Given my original configuration above and this PSU, your total comes just under $300 with shipping reaching your $300 target.

So, this post is just to show you $300 is plenty nowadays to get a good set of main components for a system. Depending on your computer needs, you can either scale up or down on the video card or cpu, but you will need to check your current PSU with the new motherboard to see if it is compatible or not. Given how old an Sempreon is, I'd advise a new PSU and not risk using the older one on newer parts.

Good luck!