Need some advice on upgrading computer. Please help!

Hello. After researching for the best PC forums on google, I found that tomshardware is the best place to seek help and advice. I'm glad I joined!

This is quite a long post, but please bare with me.

I have an old computer which I bought 4 years ago. Instead of buying a new computer, I decided to upgrade it and I found some excellent deals where I can get a good processor and GPU for less than $80. My computer's current setup is as follows:

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MOTHERBOARD: K8S-LA (Salmon)
This link gives detailed information about it: http://h10025.www1.hp.com/ewfrf/wc [...] uct=447313
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PROCESSOR: AMD Sempron 3100+ (1.8Ghz with 256kb L2 cache) Socket 754
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RAM: 1.5GB
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POWER SUPPLY: 500W
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GRAPHICS CARD: Radeon 9550, but I recently purchased a Radeon HD 2600XT AGP for a dirt cheap price of $38
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I would like to upgrade my processor as well, but I'm having a hard deciding on which. The link I provided says the following about my motherboard:

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Maximum approved processor:
Socket 754
Supports the following processors:
AMD Athlon 64 3700 processor (Clawhammer, 2.4 GHz)
AMD Athlon 64 3400 processor (Newcastle, 2.4 GHz)
AMD Sempron
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I would really appreciate it if you can answer the following questions:

1) I "might" upgrade my Sempron to an Athlon 64 3400+ (ClawHammer, 2.2Ghz with 1MB L2 cache). This processor is not listed on the list, but it is a socket 754. Will I still be able to use it?

2) Will my motherboard take any processor as long as it's socket 754?

3) Will it take socket 754 laptop processors?

4) Which processor is more powerful: The Athlon 64 3400+ (2.2Ghz with 1MB L2 cache) or the Athlon 64 3400+ (2.4Ghz with 512kb L2 cache)?

5) Will I need to upgrade my current CPU fan and heatsink considering that my Sempron and all the Athlon 64 processors are made in 130Nm with the only difference being that the Sempron's voltage is 1.40 V and consumes only 62 W, while the Athlon 64's voltage is 1.50 V and consumes 89 W. Isn't that a pretty big difference? Wouldn't that make much more heat?

6) Are the Athlon 64 processors (mainly the 3400+ and 3700+) a big step up from the Sempron?

7) I'm not looking to play the newest and highest-end PC games. I only hope to be able to play Battlefield 2 and Half-Life 2 on MAX settings. With the new HD 2600XT I just purchase combined with either an Athlon 64 3400+ or a 3700+, will be able to do so?

Last but not least, thank you so much for reading this far. I hope you can answer all my questions!

Thanks!

The resolution will be somewhere around 1024 by 768 or 1152 by 864 at most

And I'm trying to keep everything under $80 and so far I've been successful. I purchased an HD 2600XT for $38 and I plan on getting an Athlon 64 processor for $40, maybe less.

With these two upgrades I know I'll be personally set for the next year or two until I get a brand new computer