Help upgrading my pc

Edit: Sorry, obviously only for a processor. :P I'll give some suggestions later.

For dust cleaning there are a number of different methods. Compressed air and using a vacuum are some options. (Unplug your PC first, obviously.)

make sure that that cpu is compatible with your motherboard

For around a $200 budget, what's a good GPU to get? I have an AMD Radeon R9 270 currently.

Comparison...

http://cpuboss.com/cpus/Intel-Pentium-G2020-vs-Intel-Core-i3-3250

That seems like a good CPU, and I'm pretty sure it's compatible with whatever mobo you have (both the i3 and the Pentium share the same socket).

http://cpuboss.com/cpus/Intel-Pentium-G2020-vs-Intel-Core-i3-3250

If your budget is $150, I'm pretty sure you can get an AMD FX-6300 and a compatible motherboard at just around your limit. The AMD CPU could be better in your case because of the overclocking headroom, along with the multi-core performance.

http://cpuboss.com/cpus/Intel-Core-i3-3250-vs-AMD-FX-6300
http://www.newegg.com/Product/Product.aspx?Item=N82E16819113286&cm_re=fx6300-_-19-113-286-_-Product
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128565&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-PCPartPicker,%20LLC-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=

If you want to clean your PC, check out this guide.

The I3-3250 is maybe 60% faster compared to the G2020, But only if the I3 is able to use its Hyper-Threading! That means in games it is not much difference, maybe 20-30%. In my opinion ~120$ for an upgrade to I3 are wasted. Most newer games like to have a CPU with real 4 cores, so it has to be an I5-3xxx.

Get an i3-3270 instead. It's the best version of that chip.
Also, are you sure you can change your CPU? Celerons nowadays are usually integrated into the motherboard.

For dust compressed air in can or blower like http://www.amazon.com/Metro-Vacuum-ED500-500-Watt-Electric/dp/B001J4ZOAW .

I would NOT spend money on that CPU: it's not a significant upgrade over your current one. You would see no difference in every-day use, and only a small one in games.

If you can, save more money and get an i5 compatible with your motherboard. When in the future you decide to upgrade your GPU as well, the i5 wouldn't be the bottleneck the i3 will be for sure.

If it's games you want to see go better right now, look at second-hand GPUs. Even with your CPU as a bottleneck, you'd still see a bigger improvement than chaging the CPU to the i3.

Another option could be buying a 250GB SSD for O.S., programs and games. You'd get a much smoother every-day experience but no differne in games (except for halved loading times). This upgrade would easily carry over to the next PC (or 2).

About cleaning: I do it with a Hoover. I just pass it over the components using the soft tip. I take off the GPU first to better clean it. Just make sure the tip is not in some material that produces static electricity. Also make sure you disconnect the PC from power.
If you have a yard you can use, bring the PC outside and BLOW the dust away with compressed air.