eMachines EL1300G-02w Desktop - Athlon 2650e 1.6 GHz
1 x 1 GB
1.0 x 160.0 GB - Standard - Serial ATA-300 - 7200.0 rpm
NVIDIA GeForce 6150 SE
(1) You've upgrade your RAM. Good. Cheap and probably made a difference.
(2) Video. Your PC doesn't seem to have a PCIe slot for video cards. No upgrade possible
(3) Disk. Your disk is not that slow (7200 RPM vs. 5400RPM of some drives). You could get a $200 128GB SSD. Play with it on this system and plan to use it on your next one. Or buy a new larger spinning drive if you need more room.
(4) CPU. Upgrade is possible, but probably not worth the effort in this MB.
(5) Operating System. You could get WIN7 vs. XP. Some people still like XP better.
(6) Mice and keyboard. Seriously. A great mouse is nice to have, ditto keyboard.
(7) USB TV connection -- but you don't have enough CPU power for most of these.
First big change would require a new Motherboard and Processor and Memory. Since this is a small case you'd end up getting a case and power supply too. Thus the appends above saying get/build a new system. $500 + a (student?) copy of windows7 would build a very nice PC. If you got all new then your old system is avail to give away.
Second approach is to stuff a very small new MB and CPU in your existing case. Something like this ITX e-350 MB for $100. http://www.newegg.com/Product/Product.aspx?Item=N82E16813157228
It would give you a very small speed bump and end up costing you $120 for CPU, MB and new memory. Reuse the rest of your stuff. Compare the $120 you'd spend to the $350 you'd spend on a new E-350 based laptop. (This chart gives both your CPU and E-350 http://www.cpubenchmark.net/cpu_list.php
as a cross check compare to an I3-2120 low end intel at 10X the speed of your 2650e. The i3-2120 or larger would be in a new $350-$500 build.)