Firstly, going off one of the TomsHardware GPU upgrade ratings (Can't find the thread) you would not see a suitable performance upgrade doing this. Might I recommend the GTX 1050 or 1050 Ti? That would be a really good upgrade, and it costs probably around £40 extra over the 750Ti, this would be a great upgrade from a GTX 730.
From doing this, I will recommend upgrading your PSU as well, but this is not a major step, as it will require (Depending on the GPU) extra Power coming from the PSU.
If you get this one:
http://www.gigabyte.com/products/product-page.aspx?pid=6054#kf
You will need a 6 Pin Power Connector.
I will recommend a couple of good, low priced PSUs.
Such as this one:
http://pcpartpicker.com/part/xfx-power-supply-p1550sxxb9
This has the 6 Pin Power connector on it.
And changing the GPU is really easy. It will most likely be attached by a screw either inside or outside of the case (Where the VGA, DVI, HDMI or DP ports) are, and there will most likely be a latch or lever holding it into the motherboard. Just simply press that down (Do all of this gently to ensure no damage is obtained from doing this process) - Also make sure you unplug your Computer and let it sit for around a minute for it to drain of all power. You can also touch a metal casing to ground yourself from all static charge. There are many guides out there that will assist you, such as this:
http://www.pcadvisor.co.uk/how-to/pc-upgrades/how-upgrade-your-graphics-card-3470162/
You can also search for more that may be better for you.
Also, if anyone comes in and says that your Motherboard is PCI-E 2.0 and you'll lose performance, its a little white lie. I have a GTX 970 and I have no performance loss running on a PCI-E 2.0 Lane.
Although you can probably benefit from upgrading your Motherboard, RAM and Processor, as well as getting an SSD, the GPU will also be a good upgrade, it is sort of even both ways, except the GPU is the cheaper upgrade.
Depending on what games you play, the GPU upgrade will not necessarily be worth it, if you're playing on games on the settings you like, then you don't really need to upgrade, in which case I will recommend upgrading to an SSD, that will definitely help in performance, as well as maybe upgrading to 16GB of RAM, may provide a little bit of performance increase, but that is less recommended as you have 8GB already.
There are other upgrades that can help performance-wise such as this:
PCPartPicker part list:
http://pcpartpicker.com/list/63HZkL
Price breakdown by merchant: http://pcpartpicker.com/list/63HZkL/by_merchant/
CPU: Intel Core i3-6100 3.7GHz Dual-Core Processor ($110.99 @ SuperBiiz)
Motherboard: MSI B150M PRO-VDH Micro ATX LGA1151 Motherboard ($66.98 @ Newegg)
Memory: Kingston FURY 8GB (2 x 4GB) DDR4-2400 Memory ($56.99 @ Newegg)
Total: $234.96
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-11-01 10:48 EDT-0400
Which is a CPU, Motherboard and RAM upgrade. The main upgrade is the CPU.